Allow image and tag to be specified separately for target allocators

[rorynickolls-skyral]

Component(s)

collector, target allocator

Describe the issue you're reporting

Currently, the image for a target allocator can be specified with the targetAllocator.image field on the OpenTelemetryCollector custom resource.

This means that it is not possible to supply an image registry and tag separately.

An example of when this would be useful is when pulling images from a private mirror but wanting to keep the tag up-to-date automatically.

Currently, to pull images from a private registry a user must also pin the target allocator tag in their custom resource. It would be preferable to only set the registry and have the operator select the target allocator tag itself.