Support alternative fluxcd.io annotations

[squaremo]

The annotations that are interpreted by flux will need to change, since they include the domain weave.works. But we don't want to just replace them, since that will break many installations (e.g., by not ignoring something that is supposed to be ignored). So: for some fairly large window, we'll need to support both sets of annotations.

[squaremo]

Q: do we want to rewrite the old annotations to new annotations, as they are updated? Not all policies get updated by fluxd, and this might end up creating a confusion of annotations. Perhaps it would be better to use whichever were there, and deprecate the original prefix when we (inevitably) break backward compatibility, down the road a bit.