Templatized URL parameters for webhook notifications

Goal: Allow to integrate more CI/CD systems with Quay by making the webhook notification configuration more flexible. Templatize the webhook URL.

Why is this important: Today, Quay supports a limited set of template variables when construction a webhook. The only way to use them is in the JSON request body that is sent via HTTP POST to the webhook destination. This requires parsing the request body on the receiving side of the webhook, potentially complicating the integration. Some CI/CD systems, such as GitLab, make it easier to extract parameters from the URL the webhook was called with.

Acceptance criteria:

• all available template variables for webhooks can be used to parameterize the webhook target URL
• if not already available, the following template variables would need to be added
  • the name of the tag
  • the name of the image
  • the full path to the repository separate from the registry FQDN
  • the digest of the image
• the webhook target URL can be parameterized using a template syntax and the UI allows to render a preview of final target URL witch static example values