Use image digest in setters

[squaremo]

Sometimes people want to use the image digest rather than (or as well as?) the tag. Assuming https://github.com/fluxcd/image-reflector-controller/issues/87, this controller could add extra setters for the digest (e.g., sha256:abc123... of named policies (and the image ref with digest, e.g., alpine@sha256:abc123...).