Allow custom annotations to be set when pushing OCI artifacts

[stefanprodan]

Example:

  kustomize build . | flux push artifact oci://ghcr.io/org/config/app:$(git rev-parse --short HEAD) -f - \ 
    --source="$(git config --get remote.origin.url)" \
    --revision="$(git branch --show-current)@sha1:$(git rev-parse HEAD)" \
    --annotations='org.opencontainers.image.licenses=Apache-2.0' \
    --annotations='org.opencontainers.image.documentation=https://app.org/docs' \
    --annotations='org.opencontainers.image.description=Production config.'

These annotations will be available in-cluster under OCIRepository.status.artifact.metadata.