[DO NOT MERGE] Implement RFC 7523 JWT flows

[LucaButBoring]

Implements support for the RFC 7523 authentication flows. This PR is a trimmed-down version of #1020, but will likely be superceded by a future authlib-based implementation (see #1240).

Compared to #1020, this implements the flow via a separate httpx.Auth subclass, which is in-line with prior maintainer feedback on how to grow these auth implementations.

Motivation and Context

Implementation example for modelcontextprotocol/modelcontextprotocol#1046.

How Has This Been Tested?

Unit tests (TBD: unit tests for section 2.2 flow). Planning to spot test with Keycloak setup once that gets published.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[yannj-fr]

Does it means that at the client creation, we have to select a different class?

[LucaButBoring]

Does it means that at the client creation, we have to select a different class?

Yes, that's what I inferred from @ochafik's statement on #1020:

After chatting w/ the team it would seem indeed that something along the lines of #882 would reduce the maintenance complexity (changes encapsulated in httpx.Auth)