pubsub/rabbit: Allow parameters

[mytototo]

trafficstars

Is your feature request related to a problem? Please describe.

When working with pubsub packages we can pass parameters to the URL. For what I have seen and tried, it looks like it is not implemented for RabbitMQ. Why?

I would like to pass x-single-active-consumer parameter, as described here: https://www.rabbitmq.com/consumers.html#single-active-consumer

Describe the solution you'd like

Be able to set parameter like for the other pubsub packages.

Example:

OpenTopic(ctx,  "rabbit://topic?x-single-active-consumer=true")

[vangent]

Per-driver package parameters have to be explicitly supported. We just haven't done so for RabbitMQ. Can you send a PR?