feat: add assertion framework support to client authentication

[dhensby]

Summary

I'm looking to implement client assertion support. At the moment I'm leaning to keeping as much of this in
user-land code, but maybe we should have a way to register client authentication plugins (in a similar way that
we do with grant types). I think that might take quite a refactor (and breaking changes) to allow for that.

At the moment this is just a draft to check what the minimum interface is to get this working in user-land.

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Some way to inject a body parser/interpreter is needed because the assertions contain most of the relevant request data that is typically just a property in the request body and most of the library makes an assumption that all the data is props in the body.

Linked issue(s)

N/A

Involved parts of the project

Client authentication

Added tests?

todo

OAuth2 standard

• Assertion Framework for OAuth 2.0 Client Authentication Authorization Grants
• JWT profile for OAuth 2.0 Client Authentication and Authorization Grants

Reproduction

n/a