Annotate RabbitmqCluster object with version after deployment

[Zerpet]

Is your feature request related to a problem? Please describe.
The operator could have different behaviour, depending on which RabbitMQ version it has deployed. This is not possible at the moment because the operator does not know what version of RabbitMQ is deploying at any given moment.

Describe the solution you'd like
As a post-deploy step, or at any point that makes sense, the cluster-operator can consult the HTTP API to determine with precision the RabbitMQ version. With that information, it will create an annotation in the RabbitmqCluster object. Future reconcile loops can leverage this annotation if present.

Describe alternatives you've considered
Parsing a tag name is not a reliable method to determine RabbitMQ version. This information must come from RabbitMQ itself.

[MirahImage]

I can also see the annotation as being useful for users looking to determine which RabbitMQ versions they have deployed