Factor out "build" code from pgrx-pg-config

[workingjubilee]

pgrx-pg-config is better off being solely responsible for interacting with the pg_config binary, but it contains logic used in two other places, that doesn't really have anything to do with interacting with pg_config, like createdb (used in cargo-pgrx and pgrx-tests).

This has a few problems:

• Code that is actually version-independent being in pgrx-tests is slightly problematic, as that is a versioned library, and so we are potentially building the same code several times for no reason (contributing to issues like #1404). While this doesn't apply to createdb specifically, it does apply to the code invoking createdb...
• This code shared between cargo-pgrx and pgrx-tests creates a boundary where the integration code can be wrong or confused. It is probably actually better, in cases involving only 2 sites, to simply duplicate the code.
• There's barely a need for pgrx-tests to be responsible for createdb, anyways: it can use other tooling for that.

[eeeebbbbrrrr]

This code shared between cargo-pgrx and pgrx-tests needs

that seems like an incomplete thought, but...

I wonder if cargo-pgrx could benefit from a "createdb" command (probably "dropdb" too), then pgrx-tests could just shell out to PGPORT=xxxx PGDATA=xxxx cargo pgrx createdb ${test_db_name}.

I don't really see any particular harm in that since cargo-pgrx is a required build tool.