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ARROW-12050: [C++][Python][FlightRPC] Make Flight operations interruptible in Python

This uses a stop token to let interactive users interrupt a long-running Flight operation. It's not perfect: the operation won't be cancelled until the server delivers a message, so this doesn't protect against very slow servers. (In that case, we'd need some way for the stop source to call TryCancel() on the gRPC RPC object, which would be tricky.) But so long as the server is being responsive, this means Ctrl-C should do what people expect in Python.

Closes #10318 from lidavidm/arrow-12050

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>