Ensure azure_identity docs include ClientCertificateCredential

[chlowell]

#2933 removed this metadata on the assumption that docs.rs defaults to building with all features, however it actually defaults to building with none (see the docs) with the result that ClientCertificateCredential isn't documented for 0.28.0. This PR fudges the now-broken link to that type's docs so our CI runs can succeed and tells docs.rs to build with the client_certificate feature. I'll unfudge the link after the next release.

[Copilot]

Pull Request Overview

This PR fixes documentation generation for the azure_identity crate by ensuring ClientCertificateCredential is included in docs.rs builds. The issue arose when PR #2933 removed docs.rs metadata assuming all features would be built by default, but docs.rs actually builds with no features by default.

• Adds docs.rs metadata to build with the "client_certificate" feature
• Temporarily updates the ClientCertificateCredential documentation link to use a redirect URL

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
sdk/identity/azure_identity/Cargo.toml	Adds package.metadata.docs.rs configuration to enable client_certificate feature
sdk/identity/azure_identity/README.md	Updates ClientCertificateCredential link to temporary redirect URL