ssh client: add async_request method

[oliver-sanders]

Follow on from #4119

Note: Low priority.

Add an async_request method to the SSH client to match the ZMQ client.

async def async_request(self, ...)
    proc = remote_cylc_cmd(...)
    while True:
        if proc.poll():
            break
        await asyncio.sleep(self.SLEEP_INTERVAL)
    ...

This can be used in cylc scan via the get_client abstraction to support the SSH interface in this command.

Pull requests welcome!

[datamel]

Unfortunately UID has caused this to be a breaking change which needs fixing before 8.0rc1. Updating the milestone.

[oliver-sanders]

This has now become a bug, my bad.