Feature: relative url in URL datastructure

[palani-johnson]

Summary

In some of my FastAPI applications I have found the need for a relative URL function that gives you a URL that is only the path, query and fragment. I figured others might find it useful. Having it as a method on the class directly is useful in situations like Jinja templates so you dont have to pass in a helper function.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I have not updated the documentation as the URL data structure is not documented on the website.