config: Move relay port parsing into the relay module

This commit:
* creates feature/relay/relay_config.[ch],
* moves relay port parsing into them,
* exposes some code from src/app/config.c
  (we'll refactor it later in 29211), and
* adds thin wrappers to make the moved code compile.

No functional changes: the moved code is still enabled,
even if the relay module is disabled.

Part of 32213.