Specific transports could be Python package extras

[dsp-ant]

e.g., we could have mcp_python as just the base definitions, mcp_python[stdio], mcp_python[sse], etc.

[Kludex]

I think we should take a decision on this for V2. Looking at the list of packages, I'm not sure there is much benefit. For stdio, probably there is, since we would probably drop a lot of dependencies.