Fetch host from query or HTTP header "shopify-app-bridge-host" for AppBridge V2

[hoppergee]

What this PR does

Currently, AppBridge 2.0 needs a host parameter to initialize and work. But after migrating to session token, we can't easily get the host in each query. So, as inspired from #1360, we can use HTTP header to manual pass the host back to server instead of manually putting them in URL or cache in cookies.

Reviewer's guide to testing

Go check the additional concern ShopifyApp::SetShopHost which fetching host from params[:host] or the HTTP header shopify-app-bridge-host. This will help a Rails app to keep holding the host on hand through setting host in HTTP header by turbolinks or turbo and sending it back in view.

Checklist

Before submitting the PR, please consider if any of the following are needed:

• Update CHANGELOG.md if the changes would impact users
• Update README.md, if appropriate.
• Update any relevant pages in /docs, if necessary
• For security fixes, the Disclosure Policy must be followed.

[nelsonwittwer]

Sorry this took so long to get to! Since this PR was opened we have a new version of app bridge to better support JWT sessions and that is the pattern we will be supporting for the long term.