feat: health authority contract and tests

[CeceOs92]

Summary

This PR implements the Health Authority contract for registering and validating trusted health-credential issuers.

• Registers authorities with Ed25519 public keys
• Supports public-key rotation
• Supports active/suspended authority status
• Verifies registered authority status
• Validates Ed25519 authority signatures
• Adds lifecycle and RFC 8032 signature tests
• Fixes an existing Clippy duplicate-attribute warning

Validation

• cargo fmt -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test --features testutils --quiet — 109 passed

Closes #89

[vercel]

@CeceOs92 is attempting to deploy a commit to the Josie's projects Team on Vercel.

A member of the Team first needs to authorize it.