Bug 1997631 - Add telemetry for database loads and migrations to Nimbust SDK

[freshstrangemusic]

There are now two new events, nimbus_events.database_load, which is emitted every time we attempt to load the database (i.e., once per session), and nimbus_events.database_migration, which is emitted every time we attempt a database migration. These events should give us insight into how often we are experiencing database corruption leading to enrollment state being dropped.

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[freshstrangemusic]

We can't merge this until I write a PR for iOS.

[relud]

some clarifying questions, the rest lgtm