Expose RaDex Constants to Python for Testing

[MattToast]

At time of writing, we have two blocks in the test_wait_for_item.py test for the Python Dragon Client

radex/tests/python/dragon/test_wait_for_item.py

Lines 23 to 25 in 0aa1c62

	# FIXME: We should expose the poll rate through the `radex` namespace
	# rather than looking for magic env vars in the tests
	_POLL_INTERVAL = os.environ.get("RADEX_POLL_INTERVAL", 100) / 1000 # seconds

radex/tests/python/dragon/test_wait_for_item.py

Lines 53 to 55 in 0aa1c62

	# FIXME: We should expose the poll rate through the `radex` namespace
	# rather than looking for magic env vars in the tests
	poll_rate = os.environ.get("RADEX_POLL_INTERVAL", 100) # milliseconds

We should consider exposing some of these constants to the python client for testing purposes. I also could see it being useful for user application code, so maybe just exposing them in general is a good idea.