Custom annotations

[favonia]

Description

I hope there's a way to specify custom annotations in addition to the standard ones, just like the labels input. They are probably needed by ArtifactHub, and putting all the metadata configurations in one place looks more elegant.

One design decision to make is whether these custom annotations should be affected by DOCKER_METADATA_ANNOTATIONS_LEVELS. My personal preference is yes, but I can see the arguments both ways.

[crazy-max]

Yes I think that makes sense, might have been an oversight when implementing #352