Add wait event support.

[HaoyuHuang]

• pl to generate the WaitEventClass table based on https://www.postgresql.org/message-id/flat/aGKSzFlpQWSh/%2B2w%40ip-10-97-1-34.eu-west-3.compute.internal thread.

Added 3 hooks.

• pgstat_report_wait_start_hook and pgstat_report_wait_end_hook are called when pgstat_report_wait_start/end is called.
• pgstat_report_stat_hook(force) is called when the backend should flush stats to shared buffer.