identity: set up sqlite

[TheButlah]

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