Option to prune instead of GC when StorageMax is reached

Currently, the only available approach to deal with overgrowth of the repository is to basically delete everything that's not pinned.

If we equate with traditional notions from the HTTP world where basically `pinned = local` and `unpinned = cached proxied content` that approach might be considered extreme and inappropriate.

It would seem more natural to have a setting to only delete the least recently used unpinned content.

I understand that the IPFS datastore may be significantly more complex and fragmented than something like, say, the NGINX or, (on the client side) Firefox/Chrome cache store, however, unconditionally erasing everything is a very crude approach compared to how these systems operate.

In relation to #3679, I would not consider this a duplicate since I think #3679 still perpetuates the misguided idea that `cached proxied content = garbage` (why?).