Add example Snippet for PostgresColumns

[fpseverino]

Adds an example Snippet that uses PostgresRowSequence.columns to dynamically explore a database without knowing all the types upfront and links it to the DocC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.24%. Comparing base (dc5283b) to head (78d6b5b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #625      +/-   ##
==========================================
- Coverage   76.32%   76.24%   -0.09%     
==========================================
  Files         135      135              
  Lines       10207    10207              
==========================================
- Hits         7791     7782       -9     
- Misses       2416     2425       +9     

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

... and 1 file with indirect coverage changes

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[fabianfett]

Very, very good. Thanks a lot!