Add metadata for fingerprints
as requested also for conversations (https://github.com/siacs/Conversations/issues/2603), adding metadata and labels to fingerprints would make it much easier to organise and properly expire/delete them if not needed anymore. This somewhat extends request #183 (closed).
Since one key advantage of jabber is the flexibility in terms of clients, you easily end up having five or more fingerprints of your own and for each contact. It would be great to store additional info like client program or creation time, last seen timestamps to distinguish and properly expire/delete them if not needed anymore.