Expand collections API to include tag

[ekager]

For recently closed tabs we want to maintain a list of recently closed sessions (with state). I am going to reuse the collections API to do so, but need a way to separate out my under the hood implementation of collections so we don't have to show it on homescreen/don't let users edit it/ etc.
mozilla-mobile/fenix#2486

┆Issue is synchronized with this Jira Task