Add support for metafield_namespaces in webhook registration

[fourseven]

What this PR does

This is an extension on Shopify/shopify-api-ruby#1186, allowing users of the shopify_app ruby/rails library to register webhooks and expect them to return with metafields back.

Reviewer's guide to testing

I have updated the tests, as well as installing this into our codebase and testing that the app_uninstalled webhook fires, as well as an order create with metafields.

Things to focus on

1. Nothing in particular, it's only a few lines changed
2. Do you want the fixes to the tests? It was throwing an error expected positional hash (…), but received keyword arguments (…) for those two lines.

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.

[fourseven]

@nelsonwittwer could I get a review here please?

[schonert]

Nice @fourseven - This would be great! Let's get this shipped!

[lizkenyon]

Thank you for your contribution!

[lizkenyon]

I think this branch needs a rebase!

You will also need to update the changelog to match the current release. While you are updating that you could add a link to the Shopify.dev docs regarding subscribing to webhooks based on metafield namespaces.

[fourseven]

Thanks @lizkenyon I've made those changes now.