(dsc): Support credentials, `acceptLicense` and PSGallery registration

[Gijsreyn]

Summary of the new feature / enhancement

The DSC resources cannot express three things that the underlying cmdlets support and that are needed in real deployments:

1. Credentials. Register-PSResourceRepository -CredentialInfo and Install-PSResource -Credential are not modelled, so a private feed (Azure Artifacts, GitHub Packages, an internal NuGet server) cannot be used from a configuration.
2. acceptLicense. Install-PSResource -AcceptLicense is not passed, so a module that requires license acceptance fails during set with exit code 4.
3. PSGallery registration. Register-PSResourceRepository -PSGallery is a separate parameter set. Declaring { name: PSGallery } in a Repository instance on a machine where it was unregistered fails, because the script always calls Register-PSResourceRepository -Name -Uri.

Example of what this could look like:

resources:
- name: Internal feed
  type: Microsoft.PowerShell.PSResourceGet/Repository
  properties:
    name: Internal
    uri: https://pkgs.dev.azure.com/contoso/_packaging/internal/nuget/v3/index.json
    trusted: true
    credentialInfo:
      vaultName: ContosoVault
      secretName: InternalFeedPat
- name: Modules
  type: Microsoft.PowerShell.PSResourceGet/PSResourceList
  properties:
    repositoryName: Internal
    acceptLicense: true
    resources:
    - name: Contoso.Tools

Proposed technical implementation details (optional)

• Repository: add an optional credentialInfo object (vaultName, secretName) mapped to -CredentialInfo on register and set, and handle name: PSGallery without uri by calling Register-PSResourceRepository -PSGallery.
• PSResourceList: add an optional acceptLicense boolean passed to Install-PSResource -AcceptLicense, and allow credential via SecretManagement rather than plain text in the configuration.
• Never emit secrets in get, test or export output; return only the vault and secret names.