Treat segmentby columns same as compressed columns with default value

[akuzm]

This is a minor refactoring that will later allow to simplify the vectorized aggregation code. No functional or performance changes are expected.

No changes in tsbench: https://grafana.ops.savannah-dev.timescale.com/d/fasYic_4z/compare-akuzm?orgId=1&var-branch=All&var-run1=3407&var-run2=3421&var-threshold=0&var-use_historical_thresholds=true&var-threshold_expression=2.5%20%2A%20percentile_cont%280.90%29&var-exact_suite_version=false&from=now%2Fy&to=now%2Fy

Disable-check: force-changelog-file

[codecov]

Codecov Report

Attention: Patch coverage is 75.40984% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 80.88%. Comparing base (59f50f2) to head (5f88a23).
Report is 107 commits behind head on main.

❗ Current head 5f88a23 differs from pull request most recent head f97f2b0. Consider uploading reports for the commit f97f2b0 to get more accurate results

Files	Patch %	Lines
tsl/src/nodes/decompress_chunk/compressed_batch.c	76.31%	2 Missing and 7 partials ⚠️
tsl/src/nodes/decompress_chunk/exec.c	73.91%	0 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6817      +/-   ##
==========================================
+ Coverage   80.06%   80.88%   +0.82%     
==========================================
  Files         190      194       +4     
  Lines       37181    36933     -248     
  Branches     9450     9666     +216     
==========================================
+ Hits        29770    29875     +105     
- Misses       2997     3197     +200     
+ Partials     4414     3861     -553     

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[antekresic]

Looks cleaner 👍