Implement support for RETURNING clause for MERGE

[kpan2034]

This adds Timescale support for the RETURNING clause with MERGE statements, which was added in PG17.

[codecov]

Codecov Report

Attention: Patch coverage is 60.24096% with 33 lines in your changes missing coverage. Please review.

Project coverage is 92.25%. Comparing base (59f50f2) to head (1698c74).
Report is 346 commits behind head on main.

Files with missing lines	Patch %	Lines
src/import/ht_hypertable_modify.c	67.92%	17 Missing ⚠️
src/nodes/hypertable_modify.c	46.66%	16 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #7275       +/-   ##
===========================================
+ Coverage   80.06%   92.25%   +12.19%     
===========================================
  Files         190      205       +15     
  Lines       37181    38497     +1316     
  Branches     9450     9984      +534     
===========================================
+ Hits        29770    35517     +5747     
+ Misses       2997     2980       -17     
+ Partials     4414        0     -4414     

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