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.)