Expose JIT counters/timing in pg_stat_statements

Here's a patch to add the sum of timings for JIT counters to
pg_stat_statements, as a way to follow-up on if JIT is doing a good or
a bad job in a configuration.

I decided to only store the total time for the timings, since there
are 4 different timings and storing max/min/etc for each one of them
would lead to a bit too much data. This can of course be reconsidered,
but I think that's a reasonable tradeoff.

Another option I guess could be to store the max/min/etc, but only
store for the total jit time instead of for each individual one. Maybe
that'd actually be more useful?


-- 
 Magnus Hagander
 Me: https://www.hagander.net/
 Work: https://www.redpill-linpro.com/