feat (Auth Server): OAuth 2.0 Attestation-Based Client Authentication

Tracking this issue. I think the main use case for this draft spec is wallets and Verifiable Credentials.

**Abstract**
This specification defines a new method of client authentication for OAuth 2.0 [[RFC6749](https://www.ietf.org/archive/id/draft-ietf-oauth-attestation-based-client-auth-01.html#RFC6749)] by extending the approach defined in [[RFC7521](https://www.ietf.org/archive/id/draft-ietf-oauth-attestation-based-client-auth-01.html#RFC7521)]. This new method enables client deployments that are traditionally viewed as public clients to be able to authenticate with the authorization server through an attestation based authentication scheme.

[OAuth 2.0 Attestation-Based Client Authentication](https://www.ietf.org/archive/id/draft-ietf-oauth-attestation-based-client-auth-01.html)