Load OSM extension in retention background worker to drop tiered chunks

[zilder]

OSM sets a special callback for dropping tiered chunks that is used by timescaledb in ts_chunk_do_drop_chunks. But in background worker OSM library doesn't get loaded and retention policy ends up not removing tiered chunks.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.

Project coverage is 81.91%. Comparing base (59f50f2) to head (b24b4ec).
Report is 788 commits behind head on main.

Files with missing lines	Patch %	Lines
src/osm_callbacks.c	0.00%	18 Missing ⚠️
src/chunk.c	0.00%	1 Missing and 1 partial ⚠️
src/loader/loader.c	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7766      +/-   ##
==========================================
+ Coverage   80.06%   81.91%   +1.84%     
==========================================
  Files         190      247      +57     
  Lines       37181    45452    +8271     
  Branches     9450    11360    +1910     
==========================================
+ Hits        29770    37233    +7463     
- Misses       2997     3750     +753     
- Partials     4414     4469      +55     

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