Allow connecting to PostgreSQL via unix socket

[giodamelio]

Is your feature request related to a problem? Please describe.
I would like to be able to connect to PostgreSQL via a unix socket with peer auth. It is more secure and easier when not running inside Docker since there are no open network sockets needed, and no passwords (if you don't want).

Describe the solution you'd like
sqlx already supports connecting via socket (See PgConnectOptions::host()), so this would mostly be a change to the configuration system.

Additional context
I would love to contribute a patch for this, I would mostly just like guidance on the config changes.

[giodamelio]

After a bit more playing around, it seems that you can just set DEFGUARD_DB_HOST to the path /run/postgresql (or wherever your Unix Socket is located) and leave the other variables empty. Since none of them are required it seems to work fine. The key is that the path must begin with /.