Support full URL and schemeless URL when register HTTP webhooks as usual

[hoppergee]

Description

Make ShopifyAPI::Webhooks::Registry.add_registration to support below 3 types of webhook url config:

• "test-webhooks"
• "app-address.com/test-webhooks"
• "https://app-address.com/test-webhooks"

Otherwise, it may cause shopify_app gem create a wrong callback URL like https://app-address.com/https://app-address.com/test-webhooks with below config:

ShopifyApp.configure do |config|
  config.webhooks = [
    {topic: 'app/uninstalled', address: "https://app-address.com/test-webhooks", format: 'json'},
  ]
end

All developers trying to migrate to shopify_app v20 may meet this issue.

[mkevinosullivan]

Please add a entry to the CHANGELOG.md file documenting the change.

[mkevinosullivan]

A slight adjustment to the changelog entry and it's good to merge! Thank you for the contribution, as it adds flexibility to the webhooks configuration.