Support exposing to the Internet by adding SSL support

Even if it's easy to expose a Postgres cluster to the Internet, it needs to be secured.

Right now we don't care much because it's meant to be used internally only. However, it's prety useful to be able to use it from external systems that aren't connected to the app's private network. Adding and configuring wireguard is an option, but not in all cases and always requires a lot of fiddling.

Relevant information:
- https://www.postgresql.org/docs/current/libpq-ssl.html
- https://github.com/sorintlab/stolon/blob/master/doc/ssl.md

Start with the PostgreSQL libpq SSL documentation and the Stolon SSL documentation linked in the issue, then inspect the repository's cluster configuration and exposure entry points. Done means external PostgreSQL connections can be secured with SSL and the required configuration is documented and tested.

Written by the indexing model from the issue text.