Create a crypto & state store PostgreSQL backend

[jplatte]

For some bots and especially for end-to-bridge encryption capable bridges, it would be very nice if the stores could use PostgreSQL as a backend so existing infrastructure with proper backup strategies and such can be reused.

[poljar]

A contributor has started to work on this.

[poljar]

We won't have time to implement this and if somebody comes with an implementation we won't be able to maintain it ourselves.

I think it's fine for such a project to live in a separate repo. Closing as I don't wish to give the impression that we'll merge a PostgreSQL store.