Support Oauth client secret

[thomasa88]

Is your feature request related to a problem? Please describe.
I'm trying to connect to an MCP server that uses DCR and returns an Oauth client ID and client secret. The auth works fine, but oauth refresh fails due to missing client_secret. Error: invalid_client

Describe the solution you'd like
Support storing and then using client secret in refresh.

Describe alternatives you've considered
Setting up Oauth myself.

Additional context

[thomasa88]

I managed to create some working code, but it likely needs some more work: https://github.com/thomasa88/rmcp-rust-sdk/commits/oauth-client-secret/