This commit:
* creates feature/relay/transport_config.[ch],
* moves server transport config checks 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. (Some of the checks
are re-ordered, so the order of some warnings may change.)

Part of 32213.