test(http): add oauth test with header

[ngjaying]

Description

This PR adds verification tests for the OAuth2 client credentials flow in REST sink and HTTP client, specifically validating the support for custom headers (e.g. Content-Type: application/x-www-form-urlencoded) which is required for Azure Event Hubs and other providers.

Type of change

• Test cases

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.39%. Comparing base (84982d4) to head (dfbb627).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3998      +/-   ##
==========================================
+ Coverage   71.36%   71.39%   +0.03%     
==========================================
  Files         460      460              
  Lines       53105    53105              
==========================================
+ Hits        37896    37913      +17     
+ Misses      12279    12271       -8     
+ Partials     2930     2921       -9     

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