feat: support accessing PostgreSQL on unix domain sockets

[j-baker]

Fixes #4249.

A common way to deploy Postgres is to not have a port. This can simplify user management as if all users log in using Unix Domain Sockets, the OS handles authentication.

This MR allows the user to optionally provide the directory their socket is stored in.

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[brianmay]

The documentation is potentially confusing: "If this is provided, the database host, port need not be provided.", but above it says DATABASE_USER, DATABASE_PASS, DATABASE_HOST, and DATABASE_PORT are all required values.

Looking at the change, none of the DATABASE_USER, DATABASE_PASS, DATABASE_HOST, and DATABASE_PORT values are required if DATABASE_SOCKET_DIR is set.

Apart from that, assuming it works, looks good to me.

[j-baker]

I've adjusted the docs to hopefully make more sense. Can confirm that I'm running with this commit - it does work.

[JakobLichterfeld]

lgtm, ty for your contribution!

[JakobLichterfeld]

Please correct the linting issues. Please follow https://docs.teslamate.org/docs/development#code-formatting