chore (postgres): Protect the nwaku compilation process so that the user has the libpq library properly installed.

[Ivansete-status]

Background

By adding support to Postgres we've also added a dependency with the "libpq.5.so" (name on Ubuntu) library.

Details

We will add some hints in the https://docs.waku.org/ page (waku-org/docs.waku.org#78) so that the user is informed about the Postgres pre-requisite.

However, we need to enhance the ux regarding the compilation process. Therefore, we need to apply either of these:

• Avoid running the postgres-dependant modules unless the user wants to explicitly use Postgres. For that we might do sth analog to rln (when defined(rln):.) <--- This is the preferable approach.
• Have the postgres library integrated in a similar way the sqlite library is.

Related issue

#1604

[Ivansete-status]

I'm closing because we added the POSTGRES compilation flag that should explicitly be set if the user is willing to interact with Postgres