chore: use Node.js's built-in 'querystring' module

[jviide]

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Fixes #563 🦕

This pull request suggests using Node.js's built-in querystring module for serializing query strings. It's used in a way that uses %20 to encode spaces instead of Gaxios's default +.

A test for http2.ts's custom parameter serialization functionality is added. As a result the overall test coverage goes up a bit.