Add publishRate config for RabbitMQ Scaler

[rwkarg]

Adding publishRate config for RabbitMQ scaler. This is mutually exclusive with the existing queueLength config.
The use is for scaling higher throughput scenarios where there is little queue length backlog or the queue length doesn't adequately correlate to the number of instances required to match the workload.

Checklist

• Commits are signed with Developer Certificate of Origin (DCO - learn more)
• Tests have been added
• [-] A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Changelog has been updated

Fixes #

[rwkarg]

This is working in our clusters, both with a publishRate trigger and both publishRate and queueLength triggers (and the existing queueLength only ScaledObjects continue to work as before).

I'll let this bake overnight and then submit another PR with the correct DCO comment in the commit.

[rwkarg]

Closing as this is replaced with #1653