[azidentity] Allow Identity Binding mode to be set via configuration options

[clnv]

fix #25663

• The purpose of this PR is explained in this or a referenced issue.
• The PR does not update generated files.
  • These files are managed by the codegen framework at Azure/autorest.go.
• Tests are included and/or updated for code changes.
• Updates to module CHANGELOG.md are included.
• MIT license headers are included in each file.

[github-actions]

Thank you for your contribution @zarvd! We will review the pull request and get back to you soon.

[github-actions]

API Change Check

APIView identified API level changes in this PR and created the following API reviews

sdk/azidentity

[Copilot]

Pull request overview

This pull request allows the Azure Identity Binding mode configuration to be set via explicit options rather than just environment variables. The PR adds:

• A new GetFederatedToken function option to provide custom token retrieval logic
• A new AzureProxy field in WorkloadIdentityCredentialOptions to configure proxy settings explicitly
• Refactoring of internal configuration logic to separate environment variable reading from configuration application

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
sdk/azidentity/workload_identity.go	Adds GetFederatedToken and AzureProxy options to allow programmatic configuration instead of only environment variables
sdk/azidentity/workload_identity_test.go	Adds tests for the new override functionality for federated tokens and Azure proxy configuration
sdk/azidentity/internal/customtokenproxy/configuration.go	New file extracting configuration logic into a reusable Options struct with an Apply function
sdk/azidentity/internal/customtokenproxy/configuration_test.go	Comprehensive tests for the new configuration API
sdk/azidentity/internal/customtokenproxy/transport.go	Removes configuration logic moved to configuration.go
sdk/azidentity/internal/customtokenproxy/transport_test.go	Removes tests moved to configuration_test.go, adds minor whitespace cleanup
sdk/azidentity/go.work.sum	Updated Go workspace dependencies

[enj]

First pass.

[clnv]

@enj @chlowell do you think we should use EnableCustomTokenProxy instead?

[clnv]

@microsoft-github-policy-service agree company="Microsoft"