Generate image pull secret at bootstrap

[stefanprodan]

To simplify the air-gapped bootstrap we could generate the image pull secret in the same way we do for the Git secret.

Example:

flux bootstrap git \
  --registry=registry.internal/fluxcd \
  --registry-creds="username:password" \
  --image-pull-secret="regcred"

When both the --registry-creds and --image-pull-secret are specified, we'll create or update the secret using the specified credentials. If --registry-creds is specified without --image-pull-secret, we'll error out and ask the user to supply the secret name.

The flux bootstrap --registry-creds input format is the same as for flux push artifact --creds, we can reuse the parsing logic and validation from --creds.