Add HTTP verb functions

[n7studios]

Summary

Adds get(), post(), put() and delete() functions for more expressive syntax, with a view to later making make_request() an internal private function again.

Developers can instead use the above verb functions, or the upcoming additional methods we'll be adding for missing API functionality.

Testing

Existing tests pass

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)