Support CWT Proofs

[babisRoutis]

This PR introduces support for CWT proofs.

In particular, issuer metada for each supported configuration have been updated for CWT proofs to follow the LSP Potential Interoperability Specs like the example bellow.

"proof_types_supported": {
    "cwt": {
       "proof_alg_values_supported": [ -7 ],
       "proof_crv_values_supported": [ 1 ]
     }
}

Note that this is a deviation from VCI Draft 13 where in paragraph 7.2.1.3 requires that the metadata for CWT proofs should be

Cryptographic algorithm names used in the proof_signing_alg_values_supported Credential Issuer metadata parameter for this proof type SHOULD be one of those defined in [IANA.COSE.ALGS].

Furthermore the PR:

• Introduces PopSigner.Cwt to the public API to allow a caller to sign the CWT
• Introduces a CwtProofBuilder (internal class) to form the CWT and delegate signing to caller-provided PopSigner.Cwt.
• Introduces a configuration option to define the policy of using PAR endpoint

Closes #237 Closes #239 Closes #240 Closes #241