So that we can use the DIDs to prune the saved firehose stream into a nice subset! This PR doesn't have any re-checking which could be done periodically to make sure we're getting the most up to date data from the PDSes (which could also be done by listening to the account move events from the firehose).

Forgor to `git add -N` the migration files in last PR sorry!

- Add firehose.rs that processes events
- Add postgres db connection that saves the events (can turn to timescale or something silly later)