Support OAuth Dynamic Client Registration (DCR) for traditional OAuth Providers

[coezbek]

It is great that fastmcp has OAuth support now included, but in practice it is not very helpful, because the traditional OAuth providers aren't offering Dynamic Client Registration. Thus to let a client register with our MCP using Google or Github, we need to implement OAuth Proxy or OIDC Proxy as Python FastMCP does:

https://gofastmcp.com/servers/auth/oauth-proxy

https://gofastmcp.com/servers/auth/oidc-proxy

[gazzadownunder]

A functional parity port of the Python fastmcp OAuth-proxy functionality has been submitted. This also includes additional functionality to pass claims from the upstream access token to the proxy access token, this allows the proxy token to be used to provide access control.