Add `imagePullSecrets` support for Custom Resources

CRD supports `image` value only with predefined external registry.
Sometimes access to registry is restricted to internal registry only, in this case you need to define imagePullSecrets or modify serviceAccount.

Start by locating the Custom Resource definition and the code that currently handles the image and serviceAccount fields. Trace how those values reach the managed Dragonfly workload, then verify that a Custom Resource can specify imagePullSecrets for a restricted registry without changing existing behavior.

Written by the indexing model from the issue text.