Support custom FlagStringer for MutuallyExclusiveFlags

[SHIVANSHGARG07]

What type of PR is this?

• feature

What this PR does / why we need it:

Adds a Stringer field to MutuallyExclusiveFlags that lets callers
override how flags within the group are rendered in help output.
Previously, flags inside a mutex group could only use the package-level
FlagStringer, so there was no way to add group-specific context (e.g.
marking one of the flags as "required") to the help text.

• flag.go: added the FlagStringerOverrider interface
  (SetStringer(FlagStringFunc)) that flags implement to opt in to a
  custom stringer.
• flag_mutex.go: added Stringer FlagStringFunc field to
  MutuallyExclusiveFlags, plus propagateStringer() which pushes the
  group's stringer down to member flags (mirrors the existing
  propagateCategory() pattern used for Category).
• flag_impl.go: FlagBase now stores an optional per-flag stringer
  and uses it in String() when set; implements SetStringer.
• flag_bool_with_inverse.go: same treatment for BoolWithInverseFlag,
  which doesn't embed FlagBase and needed a separate implementation.
• command_setup.go: calls grp.propagateStringer() alongside the
  existing grp.propagateCategory() call during command setup.
• godoc-current.txt: regenerated to reflect the new public API.

Which issue(s) this PR fixes:

Fixes #2220

Special notes for your reviewer:

Followed the existing propagateCategory pattern for consistency.
BoolWithInverseFlag needed a standalone implementation since it
doesn't embed FlagBase.

Testing

Added TestMutuallyExclusiveFlags_PropagateStringer and
TestMutuallyExclusiveFlags_PropagateStringerNil in
flag_mutex_test.go. Ran the full suite with go test ./... — all
passing.

Release Notes

Added a `Stringer` field to `MutuallyExclusiveFlags`, allowing a custom
`FlagStringFunc` to override how flags within a mutually exclusive
group are displayed in help output.