Summary:
New changes:
- Moved wait_event.c/wait_event.h changes to pgstat.c/pgstat.h as these are the PG11 equivalent
- Removed some changes in TestYbQueryDiagnostics.java which are not present in 2024.2

Original commit: c9911097f6ba3e8455b8af8c7f9643ca7bcecdff / D41551
Background workers like pg_cron, query diagnostics have infinite loops
and they sleep for a predetermined number of milliseconds in each
iteration of the loop. This sleep can be considered idle because the
process is sleeping because doesn't have anything to do. If ASH samples
and collects these wait events, it means that the circular buffer will be
very quickly filled with idle wait events which is not ideal.

This diff ignores those idle wait events and doesn't put them in the
circular buffer.
Jira: DB-14958

Test Plan: ./yb_build.sh --cxx-test pg_ash-test --gtest_filter PgBgWorkersTest.ValidateIdleWaitEventsNotPresent

Reviewers: ishan.chhangani, hbhanawat

Reviewed By: ishan.chhangani

Subscribers: yql, svc_phabricator

Differential Revision: https://phorge.dev.yugabyte.com/D41862