Kafka producer bootstep

Addresses #385 (closed).

This is a WIP. In particular, I'm not sure if the send() method should live inside the Producer, and what other arguments it should have.

The Kafka bootstrap configuration is also currently a placeholder.

To-dos:

• Add new packages as dependencies (currently suffering from the htcondor dependency issue on Macs)
• Choose between python-confluent-kafka, adc-streaming, and hop-client (or some combination) [updated Feb 7, 2022]
• Add correct host to configuration

gwcelery/conf/playground.py

@@ -18,6 +18,12 @@ mock_events_simulate_multiple_uploads = True
 """If True, then upload each mock event several times in rapid succession with
 random jitter in order to simulate multiple pipeline uploads."""
 
+kafka_urls = {
+    'scimma': 'kafka://kafka.scimma.org/igwn.gwalert-playground',
+    'gcn': 'kafka://kafka.test.gcn.nasa.gov/igwn.gwalert'
+}
+"""Kafka broker URLs"""
+
 voevent_broadcaster_address = ':5341'
 """The VOEvent broker will bind to this address to send GCNs.
 This should be a string of the form `host:port`. If `host` is empty,