Vectorized aggregation with grouping by one fixed-size column

[akuzm]

This is a simplified implementation that uses the Postgres simplehash hash table with a generic Datum key for by-value fixed-size compressed columns.

The biggest improvement on a "sensible" query is about 90%, and a couple of queries show bigger improvements but these are very synthetic cases that don't make much sense:
https://grafana.ops.savannah-dev.timescale.com/d/fasYic_4z/compare-akuzm?orgId=1&var-branch=All&var-run1=3815&var-run2=3816&var-threshold=0.02&var-use_historical_thresholds=true&var-threshold_expression=2%20%2A%20percentile_cont%280.90%29&var-exact_suite_version=false&from=now-2d&to=now

[codecov]

Codecov Report

Attention: Patch coverage is 88.25215% with 41 lines in your changes missing coverage. Please review.

Project coverage is 82.23%. Comparing base (59f50f2) to head (cd7a1dc).
Report is 399 commits behind head on main.

Files with missing lines	Patch %	Lines
tsl/src/nodes/vector_agg/grouping_policy_hash.c	82.21%	23 Missing and 14 partials ⚠️
tsl/src/nodes/vector_agg/plan.c	75.00%	0 Missing and 3 partials ⚠️
tsl/src/nodes/vector_agg/exec.c	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7341      +/-   ##
==========================================
+ Coverage   80.06%   82.23%   +2.16%     
==========================================
  Files         190      219      +29     
  Lines       37181    39141    +1960     
  Branches     9450    10081     +631     
==========================================
+ Hits        29770    32188    +2418     
+ Misses       2997     2938      -59     
+ Partials     4414     4015     -399     

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