enhance: UrlHelper join method handle endpoint URL string

[ashleyhood]

This is to allow API endpoints to be full URLs and not have the baseUrl prepended to it. Closes issue #99

[what-the-diff]

• Added a new method to check if the url is valid
• Updated join() method to return endpoint when it's already an absolute URL
• Removed unnecessary code in join() that checks for $endpointIsNotUrl and sets glue variable accordingly, since we're now returning early from this function when endpoint is already an absolute URL (see Added tests #1)

[Sammyjo20]

Awesome, thank you!

[Gummibeer]

Thanks! 🎉🚀