Tags give the ability to mark specific points in history as being important
User-level key-value API reworked. Clients can store strings keyed by strings now.
No functional change - move artifact service into portal proto so it gets auto-documented by CI.
Steamline and unify the get materialization(s) v1 and v2 calls. This is a slight breaking change: the default response from `GetMaterialization` may be different.