Expose PostgresColumns metadata on PostgresRowSequence

[fpseverino]

Continues the work in #577

Resolves #576

[fabianfett]

Thank you!

[codecov]

Codecov Report

❌ Patch coverage is 20.00000% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.35%. Comparing base (9e9bde9) to head (2e41278).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/PostgresNIO/New/PostgresColumns.swift	14.28%	24 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #624      +/-   ##
==========================================
- Coverage   76.41%   76.35%   -0.06%     
==========================================
  Files         134      135       +1     
  Lines       10179    10207      +28     
==========================================
+ Hits         7778     7794      +16     
- Misses       2401     2413      +12     

Files with missing lines	Coverage Δ
Sources/PostgresNIO/New/PostgresRowSequence.swift	92.95% <100.00%> (ø)
Sources/PostgresNIO/New/PostgresColumns.swift	14.28% <14.28%> (ø)

... and 4 files with indirect coverage changes

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