Fail http proxy configuration when using SecItem

[xiazhvera]

Issue #, if available:

Description of changes:
Http proxy is not supported when using Apple Network Framework and will now fail during config validation.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.75%. Comparing base (a9745ea) to head (e9cdaa4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #551      +/-   ##
==========================================
+ Coverage   79.72%   79.75%   +0.02%     
==========================================
  Files          28       28              
  Lines       11894    11893       -1     
==========================================
+ Hits         9483     9485       +2     
+ Misses       2411     2408       -3     

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