Add callback to mem_guard if loaded

[mkindahl]

Add code to call mem_guard to enable memory tracking for background workers. This is enabled for all background workers started by timescaledb, but not to other workers started by PostgreSQL.

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 81.89%. Comparing base (59f50f2) to head (7180947).
Report is 804 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bgw/job.c	44.44%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7788      +/-   ##
==========================================
+ Coverage   80.06%   81.89%   +1.82%     
==========================================
  Files         190      247      +57     
  Lines       37181    45451    +8270     
  Branches     9450    11367    +1917     
==========================================
+ Hits        29770    37221    +7451     
- Misses       2997     3756     +759     
- Partials     4414     4474      +60     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[mkindahl]

This is intended as an interim solution until we have a proper plugin interface in place. See #7791 for a draft of the plugin interface.

[mkindahl]

This is slightly hard to test (in the sense, create a good regression test), but I added printouts to mem_guard and timescaledb to print out when the jobs are started and when the mem_guard function is called, and it seems to work fine.

[mkindahl]

I will create a better regression test with a separate plugin for #7791.