feat: implement signing for wallet attestation requests

[Dindexx]

Short description of what this resolves:

This PR implements the infrastructure for signing wallet attestation requests. It enables the creation of signed JWT assertions required for secure wallet attestation issuance flows within the OID4VC framework.

Changes proposed in this pull request:

• Extended IAttestationSigner: Added SignWalletAttestationRequest to the abstraction layer to support signing operations.
  Introduced SignedWalletAttestationRequest: Created a new record to encapsulate both the resulting JsonWebToken and the original request data.

• Refactored Issuance Models:

  • Moved WalletAttestationRequest and ConfirmationClaimJwk to a dedicated WalletAttestationRequest.cs file.
  • Renamed WalletAttestationIssuanceResponse to WalletAttestationResponse for better naming consistency.
  • Added Helper Methods: Implemented ToRequestEnvelope as an extension method to simplify the construction of the final request payload using the assertion-jwt field.

• Updated Constants: Added the AssertionJwt field key to WalletAttestationJsonFields.

Fixes: #

[JoTiTu]

LGTM