openhcl/attestation: Generify JWT code

[stunes-ms]

OpenHCL's JWT-handling code is currently used only for AKV key release, but some upcoming work will add more uses for it. In preparation, this change pulls the generic parts of the JWT handling into their own module.

[Copilot]

Pull request overview

This PR refactors JWT handling code to make it more generic and reusable. The specific JWT parsing and validation logic previously tied to Azure Key Vault (AKV) key release has been extracted into a new jwt.rs module to support future JWT use cases.

Key changes:

• Created new jwt.rs module with generic JWT parsing, validation, and error handling
• Refactored key_release.rs to use the new generic JWT infrastructure
• Updated Cargo.toml metadata for unused dependency tracking

Reviewed changes

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

File	Description
openhcl/underhill_attestation/src/lib.rs	Added jwt module declaration
openhcl/underhill_attestation/src/jwt.rs	New generic JWT module with parsing, signature verification, and certificate chain validation
openhcl/underhill_attestation/src/igvm_attest/key_release.rs	Refactored to use generic JWT helper instead of AKV-specific implementation
openhcl/underhill_attestation/Cargo.toml	Updated unused-deps metadata

[chris-oo]

Mainly looks fine just a few comments