Replace httpx with httpx2 in examples

[timkpaine]

Description

Replace httpx with httpx2 in ETL examples, related documentation, and example test dependencies.

The four non-CLI ETL example tests pass. The CLI test remains blocked by the existing missing Hydra callable/extract config, before HTTP code runs.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup
• CI / build configuration
• Other (describe below)

Checklist

• Linting passes (Ruff on changed Python)
• Tests pass (four non-CLI ETL example tests)
• New tests added for new functionality
• Documentation updated (if applicable)
• Changelog / version bump (not applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.48%. Comparing base (6a788a6) to head (bee98de).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #263   +/-   ##
=======================================
  Coverage   93.48%   93.48%           
=======================================
  Files         176      176           
  Lines       20327    20327           
  Branches     1350     1350           
=======================================
  Hits        19002    19002           
  Misses       1052     1052           
  Partials      273      273           

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