Support multiple worker 'types'

[jonnydford]

Add support for multiple worker types. For example, a high memory worker and a 'default' worker.

A couple of ways this could be implemented:

• An option for an AirflowCluster to be worker only, i.e. no flower, ui, scheduler and attach to the same base. Manually set your Redis?
• An AirflowWorker CRD so you can define them multiple times
• A slice of []Workers with AirflowCluster