Attestation-based client authentication support

[dtsiflit]

Description of change

PR aims to provide support attestation-base client (wallet) authentication.

• New enum named Client having two cases public and attested. Client would express the OAUTH2 client and in particular the way that it can be authenticated to token and/or PAR endpoint
• Assume that the attested case has already a ClientAttestationJWT. How this is provisioned is outside the scope of the PR
• Replace clientId with client in OpenId4VCIConfig
• Take into account Wallet Attestation Schema from HAIP

Closes #75

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the readme
• My changes generate no new warnings
• I have added unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes