Expose expires on in attestation token #5739

[gladjohn]

Fixes #5739

Changes proposed in this request
This pull request enhances the attestation result returned by the attestation client to include structured token expiry information, making it easier for caching based on the expiration of attestation JWTs without manual decoding. The main changes introduce a new AttestationToken type, extract expiry from JWTs, and update code and tests to use the new structure.

Attestation result structure and expiry extraction:

• Introduced a new AttestationToken record that encapsulates the raw JWT and its expiration (ExpiresOn), allowing structured access to expiry metadata.
• Updated the AttestationResult record to include the new Token property, and updated all usages and documentation to reflect this addition. The original Jwt string is retained for backward compatibility. [1] [2]
• Implemented a new JwtClaimExtractor utility class to extract the exp (expiration) claim from JWT payloads using existing MSAL utilities.

Attestation client and error handling updates:

• Modified AttestationClient.Attest to extract the expiry from the JWT, populate the new AttestationToken, and update all error paths to use the new result structure. [1] [2] [3]
• Updated error handling in PopKeyAttestor and unit tests to construct AttestationResult with the new Token property, ensuring consistency across the codebase. [1] [2]

Testing
Updated existing tests.

Performance impact
none

Documentation

• All relevant documentation is updated.