Client implementation of MCP auth

[jspahrsummers]

To do

• Dynamic client registration
• Tests
• Add way to provide client ID (and secret?) to authorization flow
• Combine all the logic into convenient entry point(s)
• Add conveniences to SSEClientTransport initialization/start for auth
• Incorporate into inspector
• Add MCP-Protocol-Version header
• Figure out what to do when hitting a 401 during POST, rather than SSE connection open
• Tests for SSEClientTransport + auth