identity: set up sqlite

This PR un-stubs the `/create` and `/users/:id/did.json` endpoints by implementing a database via sqlx + sqlite. In particular, I implemented: * SQL schema migration, including validation that all migrations on the db are up to date * Automatic db file creation if it doesn't exist * No need for hosting a db yourself, since sqlite is statically linked and runs on same node * support for controlling db file path via CLI or env var