chore: protected TwilioRestClient constructor

[bcanseco]

Fixes

This PR changes the access modifier for the TwilioRestClient constructor from private to protected. Allows for extending the class (e.g. for wrapping the super.request() method with instrumentation).

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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• 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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