Add OAuth authentication client for HTTPX

[ihrpr]

Adding support for Auth in client using HTTPX Custom authentication schemes
.

NOTE: This PR adds support for Streamable HTTP, but SSE will be added as a follow up, all we need to do is to add one parameter.

This PR Implements Auth in spec version 2025-03-26

• Implementation supporting dynamic client registration, authorization flow, and automatic token refresh
• Pluggable Storage System - Support for different token storage backends (in-memory and file-based providers included)
• Streamable HTTP Integration - OAuth authentication integrated with the existing streamable HTTP transport
• Example Client - A complete working example demonstrating OAuth authentication flow with an interactive CLI

Follow ups:

• add support for SSE

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Related Issues:
#652
Related PRs:
#700
#308

[pcarleton]

minor nits, i also pushed a commit for scope handling.

I tested this against the typescript example, and it successfully connected after the scope fix

[pcarleton]

think it's upset about some linting errors. but otherwise lgtm!

[aravind-segu]

@ihrpr Thank you very much for working on this request quickly! The OAuth implementation looks great and works well for our use case. With the Token Storage Provider in place, as long as the contract is maintained, we can also bypass the 3-legged OAuth flow and connect to MCP servers using an internally generated OAuth token - which is also perfect for us.