fix(cubesql): Fix Postgres meta layer schemas and data

[MazterQyou]

Check List

• [x] Tests has been run in packages where changes made if available
• [x] Linter has been run for changed code
• [x] Tests for the changes have been added if not covered yet
• [x] Docs have been added / updated if required

Description of Changes Made

This PR fixes Postgres meta layer tables' column types and results, improving future compatibility. Meta layer has been refactored to exclude unsigned types as those are not part of PostgreSQL. All unsigned types have been replaced with signed equivalents.

[codecov[bot]]

Codecov Report

Base: 82.27% // Head: 73.64% // Decreases project coverage by -8.62% :warning:

Coverage data is based on head (e8c2003) compared to base (52877ac). Patch coverage: 94.34% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4572      +/-   ##
==========================================
- Coverage   82.27%   73.64%   -8.63%     
==========================================
  Files         126      264     +138     
  Lines       18861    30063   +11202     
  Branches        0     2791    +2791     
==========================================
+ Hits        15517    22141    +6624     
- Misses       3344     7623    +4279     
- Partials        0      299     +299     

Flag	Coverage Δ
cube-backend	59.77% <ø> (?)
cubesql	82.17% <94.34%> (-0.10%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...gine/information_schema/postgres/character_sets.rs	93.10% <ø> (-0.12%)	:arrow_down:
...ne/information_schema/postgres/pg_stat_activity.rs	0.00% <0.00%> (ø)
...cubesql/cubesql/src/compile/rewrite/rules/split.rs	96.96% <ø> (+0.64%)	:arrow_up:
...src/sql/database_variables/postgres/global_vars.rs	0.00% <0.00%> (ø)
rust/cubesql/cubesql/src/sql/postgres/extended.rs	92.66% <ø> (ø)
rust/cubesql/pg-srv/src/lib.rs	66.66% <ø> (ø)
rust/cubesql/cubesql/src/sql/types.rs	52.00% <40.00%> (+1.01%)	:arrow_up:
.../compile/engine/information_schema/postgres/ext.rs	69.23% <57.89%> (-30.77%)	:arrow_down:
rust/cubesql/cubesql/src/sql/postgres/pg_type.rs	62.50% <83.33%> (+8.84%)	:arrow_up:
rust/cubesql/cubesql/src/compile/mod.rs	80.06% <88.88%> (-0.03%)	:arrow_down:
... and 200 more

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