Provide options for an external Postgres

### Problem Statement

As a user I prefer to run myself the Postgres and Clickhouse DBs.
It also makes me sure the DB servers still run when I down/up Sentry.

A lot of the setup relies onto having Postgres as a container for upgrades.
Can an external option be done ?
Same for Clickhouse but much less a priority.

### Solution Brainstorm

Currently I comment out the Postgres container and tweaks envs.
And also have to comment all your upgrade lines, and check on each new release. (this is quite the most problematic)

Solution: ask Sentry itself for a DB connection with Python and run the migration ?

Start by reviewing the self-hosted Postgres container configuration, environment settings, and upgrade steps described in the issue. Determine how an external Postgres connection and migrations would fit the existing setup; done means users can run Postgres outside the stack without repeatedly commenting out containers or upgrade lines.

Written by the indexing model from the issue text.