This commit adds two Glean pings to the Storage-Sync browser component:
one for history, and one for bookmarks. Both pings record the same set
of base metrics, including incoming and outgoing counts, sync duration,
the hashed FxA UID, and, most importantly, the failure reason if the
store fails to sync. The bookmarks ping records additional validation
data.

The Glean schema is a flattened version of the Sync ping that's
currently sent on Firefox Desktop and Firefox for iOS. Instead of
sending a single ping with multiple syncs, each having multiple
engines, we send one Glean ping per engine per sync.

The pings are recorded directly in the component, so any app that
consumes it should get pings for free.