Remove circular #include's between wait_event.h and wait_event_types.h

Remove circular #include's between wait_event.h and wait_event_types.h

wait_event_types.h is generated by the code, and included wait_event.h.
wait_event.h did the opposite move, including wait_event_types.h,
causing a circular dependency between both.

wait_event_types.h only needs to now about the wait event classes, so
this information is moved into its own file, and wait_event_types.h uses
this new header so as it does not depend anymore on wait_event.h.

Note that such errors can be found with clang-tidy, with commands like
this one:
clang-tidy source_file.c --checks=misc-header-include-cycle -- \
  -I/install/path/include/ -I/install/path/include/server/

Issue introduced by fa88928470b5.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/350192.1745768770@sss.pgh.pa.us