Implement outbound MCP authentication

[alan-anzenna]

As there's a TODO in the code, I'm sure you know about this, but we'd like an issue that we can follow on github.

In mcp_tool.py, there's the following code:

    # TODO(cheliu): Support passing auth to MCP Server.
    self._auth_scheme = auth_scheme
    self._auth_credential = auth_credential

The _auth_* fields are initialized, but then never used.

This appears to be another request for this in a different issue. However, that issue appears to be about something else, and the linked PR does not address the issue.

[omarbenhamid]

Seems that Client OAuth support was just merged in the MCP Python SDK but after the 1.9.1 release, I guess it will be released in the next version. I hope it will not take long.