feat: Enable cancellation during and before a stream is established

[kthui]

Overview:

• Python binding will cancel the outgoing request if cancellation happens before the stream is established.
• Migration will abort creating new streams if the request is cancelled.

Details:

• Python binding update to allow context to be linked before making the outgoing request.
• New unit test checking the cancellation behavior with Python binding.
• Migration logic enhancement to support aborting new stream construction if request is cancelled.
• New unit test checking the Migration new stream construction abort logic.

Where should the reviewer start?

Start with the Python binding unit tests, and then the Python binding changes, and finish off with the Migration layer.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

N/A