docs: explain the HTTP client adapter requirement

[zahidgh]

Summary

• document the HTTPlug/PSR-18 adapter requirement immediately after installation
• show supported Composer commands for cURL and Guzzle 7 adapters
• add Composer suggestions so the missing transport dependency is visible during installation

Without an adapter, the client cannot discover an HTTP transport when a provider request is created.

Validation

• composer validate --no-check-publish --no-interaction
• git diff --check

Fixes #82

[github-actions]

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Co-authored-by: zahidgh <zahidui@git.wordpress.org>
Co-authored-by: Ref34t <mokhaled@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.60%. Comparing base (0a9a1b3) to head (0d4b8dd).

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk     #278   +/-   ##
=========================================
  Coverage     86.60%   86.60%           
  Complexity     1335     1335           
=========================================
  Files            68       68           
  Lines          4314     4314           
=========================================
  Hits           3736     3736           
  Misses          578      578           

Flag	Coverage Δ
unit	86.60% <ø> (ø)

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