Clean up spaghetti code in ExecutorCheckPerms_hook

[jeltz]

The code in our ExecutorCheckPerms_hook was almost impossible to read so cleaned it up so people can understand it. Some of the changes include:

• Use argument names from PostgreSQL
• Use REL_LST as array size instead of 20
• Use RELKIND_VIEW instead of 'v'
• Reduce the number of scopes'
• Add a couple of comments

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.45%. Comparing base (ac4a4f7) to head (52babae).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pg_stat_monitor.c	90.90%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #660      +/-   ##
==========================================
+ Coverage   86.31%   86.45%   +0.13%     
==========================================
  Files           3        3              
  Lines        1345     1344       -1     
  Branches      218      218              
==========================================
+ Hits         1161     1162       +1     
+ Misses         87       86       -1     
+ Partials       97       96       -1     

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