gRFC L54: Adding a "wait_for_termination" API for Python server

[Lidi Zheng]

gRFC: https://github.com/lidizheng/proposal/blob/python-wait/L54-python-server-wait.md

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In short, Python server only provided a non-blocking "start" function and didn't provide a method to prevent main thread from exiting.

This gRFC is trying to introduce a simple method named "wait_for_termination" which will block until the server is stopped/terminated.

Feel free to comment :)