Add api_host to ShopifyAPI::Context

[brendo]

RE: #1235

This PR adds new api_host "setting" to Context. The HttpClient will use api_host when set, otherwise default to calling the API as specified by the session.shop value.

Whilst this feels a little weird to add this value to Context, it seems to be way the ShopifyAPI gem is "configurable" by consumers.

How has this been tested?

Unit tests in this project and using this branch in a service.

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.

[nelsonwittwer]

Thanks for taking the time to do this!