messagequeue: Extract listen_messages from consume_messages

Give a way for developers to iter their development queues. For example, following messaging docs, a dev can open an ipython shell and do:

from cki_lib import messagequeue
queue = messagequeue.MessageQueue()
listener = queue.listen_messages(
    exchange="cki.exchange.webhooks",
    routing_keys=["gitlab.com.cki-project.#.issue"],
)
message = next(listener)
print(message.body)

To experiment with one message at a time with no need of a callback function.

Relates to: infrastructure#854

See also: documentation!583

Signed-off-by: Tales da Aparecida tales.aparecida@redhat.com