Allow the imgix swift library to have initializer that accepts no host parameter

[AbuBoo]

Description:
When using the Swift Library to construct imgix URLs, you must include a host to initialize the imgix client. An image of the code block is included below.
https://assetmanager.imgix.net/Testing-%F0%9F%98%83.png

The suggestion would be to allow these URLs to be built using an input URL. Similar to what is being used in our current URL reconstruction library.
https://assetmanager.imgix.net/URlRecon.png

Problem:
Currently, Grailed is trying to use our library to build the URLs they will be listing in their IOS app using the Swift library. They are building these URLs using custom domains. So when building the host URL they would rather the client builder use the host that's already in the URL.

Suggested Solution:
To allow the URLs to accept a no-host input and grab the host name from the input URL.

Describe alternatives you've considered
For now, they can just use the imgixClient instance or use the URL reconstructor to ensure they get the correct URL.