Add OAuth authentication client for HTTPX

[dsp-ant]

Summary

• Adds OAuth 2.1 authentication client implementation with PKCE support
• Implements HTTP authentication for the HTTPX client
• Supports dynamic client registration, token refresh, and authorization code flow
• Provides in-memory token storage implementation

[Jflick58]

@dsp-ant do you want any assistance on this? Happy to jump in if needed. Really excited to see this implemented!

[smurching]

@dsp-ant I had a similar question to @Jflick58, this PR is super exciting & seems key to being able to authenticate to remote MCP servers via the Python SDK - happy to help here if it's useful, & also curious if there are any known blockers/target timeline to add this support :)

cc @saflamini

[ihrpr]

closing this as #751 builds on top