chore: protected NetworkHttpClient field

[bcanseco]

Fixes

This is a similar PR to #631.

This PR changes the access modifier for the NetworkHttpClient's client field from private to protected. Allows for extending classes to use this overridden field (e.g. if you'd want your own custom NetworkHttpClient to have a request method that sends as JSON instead of URL-encoded, but want everything else to be the same).

Verified that make test passes but haven't added any additional tests since it's just a simple access change / would just be testing a core Java feature. Lmk if we do want a test for this and I'd be happy to write something up.

Checklist

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• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
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• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
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• [ ] I have added inline documentation to the code I modified

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[Hunga1]

Just need to update the branch with the base.