Allow users to specificy a maximum spec tag count

As discussed here:
https://coreos.slack.com/archives/CB95J6R4N/p1631564419212700?thread_ts=1630435609.052600&cid=CB95J6R4N

There are use cases where a user or component might grow the set of spec tags each release.

Rather than making such components responsible for their own management/pruning of spec tags to ensure the total number of tags in the imagestream stays under our recommended limit of ~100, it would be useful if it was possible to annotation an imagestream with a max spec tag count and have our existing imagestream controller logic automatically prune away the oldest (by order in the spec array) tags over the specified limit.

cc ccoleman1@redhat.com obulatov@redhat.com