feat: use opaque memory in HTTP APIs

[martintmk]

Summary

• change HttpBodyBuilder to store OpaqueMemory while accepting any MemoryShared provider
• make OpaqueMemory::new preserve an existing OpaqueMemory instead of adding another wrapper
• change fetch dependencies to expose OpaqueMemory, allowing customers to choose their memory provider
• keep GlobalPool as the default provider and retain the existing bytesbuf type names

Validation

• full workspace clippy with all targets and features
• full workspace tests with all features
• affected-package formatting, no-default-features build, and external-type checks
• repository spelling and generated README checks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (a6ca774) to head (ceac638).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #684    +/-   ##
========================================
  Coverage   100.0%   100.0%            
========================================
  Files         554      555     +1     
  Lines       61509    60642   -867     
========================================
- Hits        61509    60642   -867     

Flag	Coverage Δ
linux	89.3% <100.0%> (-10.7%)	⬇️
linux-arm	89.1% <100.0%> (-10.9%)	⬇️
scheduled	?
windows	90.0% <100.0%> (?)

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[github-actions]

⚠️ Potential breaking changes detected

cargo semver-checks flagged the following on this PR. This is informational -- breaking changes between commits are expected; the major-version bump happens at release time, not on every PR.

bytesbuf

     Cloning origin/main
    Building bytesbuf v0.8.0 (current)
       Built [   3.236s] (current)
     Parsing bytesbuf v0.8.0 (current)
      Parsed [   0.004s] (current)
    Building bytesbuf v0.8.0 (baseline)
       Built [   3.095s] (baseline)
     Parsing bytesbuf v0.8.0 (baseline)
      Parsed [   0.005s] (baseline)
    Checking bytesbuf v0.8.0 -> v0.8.0 (no change; assume minor)
     Checked [   0.017s] 196 checks: 195 pass, 1 fail, 0 warn, 49 skip

--- failure method_requires_different_generic_type_params: method now requires a different number of generic type parameters ---

Description:
A method now requires a different number of generic type parameters than it used to. Uses of this method that supplied the previous number of generic types will be broken.
        ref: https://doc.rust-lang.org/reference/items/generics.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/method_requires_different_generic_type_params.ron

Failed in:
  bytesbuf::mem::OpaqueMemory::new takes 1 generic types instead of 0, in /home/runner/work/oxidizer/oxidizer/crates/bytesbuf/src/mem/opaque_memory.rs:34

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [   7.379s] bytesbuf

fetch

     Cloning origin/main
    Building fetch v0.15.0 (current)
       Built [  27.386s] (current)
     Parsing fetch v0.15.0 (current)
      Parsed [   0.006s] (current)
    Building fetch v0.15.0 (baseline)
       Built [  26.054s] (baseline)
     Parsing fetch v0.15.0 (baseline)
      Parsed [   0.006s] (baseline)
    Checking fetch v0.15.0 -> v0.15.0 (no change; assume minor)
     Checked [   0.012s] 196 checks: 194 pass, 2 fail, 0 warn, 49 skip

--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/constructible_struct_adds_field.ron

Failed in:

  field CustomDeps.memory in /home/runner/work/oxidizer/oxidizer/crates/fetch/src/custom.rs:56

     Summary semver requires new major version: 2 major and 0 minor checks failed
--- failure struct_pub_field_missing: pub struct's pub field removed or renamed ---
    Finished [  55.016s] fetch

Description:
A publicly-visible struct has at least one public field that is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/struct_pub_field_missing.ron

Failed in:
  field global_pool of struct CustomDeps, previously in file /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/2015f49773bae6feddac285d64bfecd2c655a4b2/crates/fetch/src/custom.rs:56

http_extensions

     Cloning origin/main
    Building http_extensions v0.9.0 (current)
       Built [   7.772s] (current)
     Parsing http_extensions v0.9.0 (current)
      Parsed [   0.004s] (current)
    Building http_extensions v0.9.0 (baseline)
       Built [   7.832s] (baseline)
     Parsing http_extensions v0.9.0 (baseline)
      Parsed [   0.004s] (baseline)
    Checking http_extensions v0.9.0 -> v0.9.0 (no change; assume minor)
     Checked [   0.016s] 196 checks: 195 pass, 1 fail, 0 warn, 49 skip

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/inherent_method_missing.ron

Failed in:
  HttpBodyBuilder::with_custom_memory, previously in file /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/2015f49773bae6feddac285d64bfecd2c655a4b2/crates/http_extensions/src/body/builder.rs:96

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  16.513s] http_extensions