Introduce RemoteAuthProvider for cleaner external identity provider integration, update docs

[jlowin]

This enhancement introduces RemoteAuthProvider, a new base class that simplifies integration with external identity providers by composing token verification with OAuth discovery metadata. This creates standardized OAuth 2.0 Protected Resource endpoints while maintaining clean separation of concerns.

The new architecture makes it easier to build authentication providers that work with external identity services. RemoteAuthProvider handles OAuth metadata generation and MCP protocol integration, while delegating token validation to configurable underlying verifiers.

New RemoteAuthProvider pattern:

from fastmcp import FastMCP
from fastmcp.server.auth.providers.jwt import JWTVerifier
from fastmcp.server.authimport RemoteAuthProvider

token_verifier = JWTVerifier(
    jwks_uri="https://auth.example.com/.well-known/jwks.json",
    issuer="https://auth.example.com"
)

auth = RemoteAuthProvider(
    token_verifier=token_verifier,
    authorization_servers=["https://auth.example.com"],
    resource_server_url="https://your-mcp-server.com"
)

mcp = FastMCP(auth=auth)

Key enhancements:

• New RemoteAuthProvider base class composes token verification with OAuth metadata
• AuthKitProvider refactored to inherit from RemoteAuthProvider for better architecture
• Consolidated resource server URL configuration across authentication patterns
• Streamlined HTTP server authentication integration with cleaner get_routes() pattern
• Enhanced documentation with clearer guidance on authentication responsibility levels

The changes maintain full backward compatibility while providing a cleaner foundation for building custom external identity provider integrations.

[taylorwilsdon]

Any plans to tie this in more directly with the session context?