Use arrow cache only for index scans

[erimatnor]

Since compressed segments are accessed sequentially during sequence/columnar scans, it is not necessary to cache the decompressed arrow arrays.

Instead, use the arrow cache only if an index scan is detected. Also make it possible to manually disable the cache by setting the GUC hypercore_arrow_cache_max_entries to 0.

[codecov]

Codecov Report

Attention: Patch coverage is 86.95652% with 6 lines in your changes missing coverage. Please review.

Project coverage is 81.95%. Comparing base (59f50f2) to head (c855d77).
Report is 783 commits behind head on main.

Files with missing lines	Patch %	Lines
tsl/src/hypercore/arrow_tts.c	87.50%	0 Missing and 5 partials ⚠️
tsl/src/hypercore/arrow_array.c	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7724      +/-   ##
==========================================
+ Coverage   80.06%   81.95%   +1.88%     
==========================================
  Files         190      247      +57     
  Lines       37181    45460    +8279     
  Branches     9450    11363    +1913     
==========================================
+ Hits        29770    37256    +7486     
- Misses       2997     3730     +733     
- Partials     4414     4474      +60     

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