Introduce WAIT_EVENT_EXTENSION and WAIT_EVENT_BUFFER_PIN

Hi hackers,

Please find attached a patch to $SUBJECT.

This is preliminary work to autogenerate some wait events
code and documentation done in [1].

The patch introduces 2 new "wait events" (WAIT_EVENT_EXTENSION
and WAIT_EVENT_BUFFER_PIN) and their associated functions
(pgstat_get_wait_extension() and pgstat_get_wait_bufferpin() resp.)

Please note that that would not break extensions outside contrib
that make use of the existing PG_WAIT_EXTENSION.

[1]: https://www.postgresql.org/message-id/flat/77a86b3a-c4a8-5f5d-69b9-d70bbf2e9b98@gmail.com

Looking forward to your feedback,

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com