[WIP] Add new method to construct host app URL

[mkevinosullivan]

Description

For performance soon we will be recommending that apps favour server redirects within the OAuth process. This will replace client side redirects using AppBridge. We want to control that code that constructs the URL for the app. That way it's easier for developers, and if that URL changes we can update the logic with a version bump.

This utility will be used to get the URL to redirect to when the store does not have a token for the app.

Ruby API equivalent of Shopify/shopify-api-js#419

What is this pull request doing?

Add a new utility called ShopifyAPI::Utils::AppUrl.getEmbedded. This utility constructs the host URL for an embedded app.

How has this been tested?

Unit tests added to test green path as well as error paths.

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.