Implement COSE Signing Endpoint with Validation and Error Handling (Acceptance Criteria)

[dharanitharan27]

This pull request addresses [DMP-2025]: Key manager - COSE Signing and Verification API (#280) by implementing the .
../keymanager/coseSign endpoint with the following features:

• Endpoint Availability: The endpoint accepts HTTP POST requests.
• Request Parsing and Validation: Validates the request structure and required fields.
• Base64URL and CBOR Decoding: Decodes input fields as per the API requirements.
• Key Selection: Dynamically selects the correct signing key based on the kid parameter in the request.
• COSE_Sign1 Signature Generation: Generates a valid COSE_Sign1 signature using the appropriate key and algorithm.
• Response Structure: Returns a response with the coseSignedData field containing the Base64URL-encoded COSE_Sign1 structure.
• Error Handling: Handles invalid requests and cryptographic errors gracefully, returning appropriate error details in the errors array.
• External Verification: Ensures the generated coseSignedData can be decoded and verified externally using standard COSE libraries.

Additional Notes:

1. Unit tests for cryptographic operations are included.
2. Integration tests for the /coseSign endpoint should be reviewed or added to ensure full coverage.
3. This implementation adheres to the defined API structure and acceptance criteria.

Please review and provide feedback.