Add OAuth 2.0 Authentication Support

[NiharDoshi99]

Is your feature request related to a problem? Please describe.

The modelcontextprotocol/go-sdk currently does not provide any built-in support for OAuth 2.0 authentication, which is required for connecting to MCP servers that enforce modern authentication flows.

[jba]

We have experimental support, protected by a build tag. See https://github.com/modelcontextprotocol/go-sdk/blob/main/auth/client.go.

[findleyr]

Closing as a dupe of #19.