protonmsg: allow users to specify router-specific topic prefixes

Prior to this change, Koji would always send messages with a hard-coded
topic:// prefix.

This works fine for ActiveMQ 5 and Artemis, but RabbitMQ does not
support that syntax. Instead, RabbitMQ brokers expect clients to use
topic addresses with a "/topic/" prefix.

The RFE for RabbitMQ to support "topic://" prefixes is
https://github.com/rabbitmq/rabbitmq-server/issues/2583

In the meantime, allow users to specify "topic://" or "/topic/"
explicitly in their configuration.

For backwards-compatibility, if the user chose neither "topic://" nor
"/topic/", prepend the "topic://" string, preserving the plugin's
existing behavior.

(Note: ActiveMQ 5 advertises its expected topic:// prefix in the initial
connection properties, so we could read that value dynamically, but
RabbitMQ and Artemis do not send an expected topic prefix connection
properties, so we just make the user choose explicitly here.)

docs/source/plugins.rst

@@ -197,7 +197,11 @@ The following fields are understood:
 * ``cert`` -- the combined client cert and key file for authenticating koji to
   the broker.
 * ``cacert`` -- the CA certificate to verify the broker server TLS connection
-* ``topic_prefix`` -- this string will be used as a prefix for all message topics
+* ``topic_prefix`` -- Koji uses this string as a prefix for all message
+  topics. For example, if you choose ``topic://koji``, then Koji
+  will publish messages on ``topic://koji.package.add`` when an user runs
+  ``kojidev add-pkg`` etc. Use ``topic://`` prefixes for ActiveMQ brokers,
+  ``/topic/`` for RabbitMQ brokers.
 * ``connect_timeout`` -- the number of seconds to wait for a connection before
   timing out
 * ``send_timeout`` -- the number of seconds to wait while sending a message