Add Postgres check

[alexliesenfeld]

It would be useful to have a library of commonly used health checks. In the scope of this issue, a health check should be implemented that checks if Postgres is alive (e.g. calling an sql.PingContext first and do a 'SELECT VERSION()' afterwards to see if Postgres is ready for query execution.

A few constraints to keep things clean and simple:

• As with all health check functions of this library, the check implementation should support accepting and forwarding context.Context that is being passed to each health check function.
• If the health check requires external external modules or other dependencies, the check should be implemented as a separate Go module(similarly how it is done for the tests, that require dependencies to other packages as well). This is required to avoid that when people import the base library, they also need to import all dependencies of all provided checks.
• The check should have a README.md in its root directory that describes how to use it and maybe what to look out for.
• You should provide a test that checks at least the happy path. If a docker container is required to properly test the check, the docker container can be added to the docker-compose.yaml file in the checks base package.

[alexliesenfeld]

Resolved with commit 12487da.