Add getTagsForObjects in ITags

Returns the list of tags that are set on the given object ids.

Required for the "favorites/tags" task (#2368)  but also a nice addition to the API.
See https://github.com/owncloud/core/pull/12360/files#diff-471be045dd1eb605d9a1197b9ba90560R52

Please review @ockham @LukasReschke @icewind1991 @DeepDiver1975 
not worth it, executequery() internally creates a prepared statement because that implements the logic to replace questionmarks with the correctly escaped value.

if you really want to make this faster you should reduce the number of trips to the db. have a look at the search query in the filecache that does chunks of 1000 with sql IN () to not break mysql.