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.)
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