Feat: Support for RFC 7521 Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants

[RFC-5721 Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants](https://datatracker.ietf.org/doc/html/rfc7521) defines:

```
This specification provides a general framework for the use of
assertions as authorization grants with OAuth 2.0. It also provides
a framework for assertions to be used for client authentication. It
provides generic mechanisms for transporting assertions during
interactions with an authorization server's token endpoint as well as
general rules for the content and processing of those assertions.
The intent is to provide an alternative client authentication
mechanism (one that doesn't send client secrets) and to facilitate
the use of OAuth 2.0 in client-server integration scenarios, where
the end user may not be present.
```