Allow setting annotations on VolumeSnapshots

[timstoop]

Describe the feature you'd like to have.

We would like to be able to set standard annotations on VolumeSnapshots. Currently, only setting labels is allowed.

What is the value to the end user? (why is it a priority?)

Being able to set annotations allows for better integration with other tools. For instance, when you only want to keep VolumeSnapshots around for at most 60 days, you could set an annotation that is picked up by kube-janitor to automatically remove the snapshot after it's 60 days old. I can imagine other tools exist that would be able to make use of annotations instead of labels.

How will we know we have a good solution? (acceptance criteria)

Annotations can be part of the VolumeSnapshotTemplate.

Additional context

In our case, we already use kube-janitor a lot and we only want to keep snapshots around for at most 30 days. Although I can imagine some people would prefer to have that expiration option build into snapscheduler, I think it would be easier to implement allowing annotations in the VolumeSnapshotTemplate instead, as I can imagine that would solve other issues as well and kinda makes sense as well to allow it when labels are also allowed.