Fix authorization webhook implementation and add tests

[tsipinakis]

Changes introduced with this PR

The authorization webhook had an incomplete implementation leading to it not functioning at all. Additionally, the configuration options were not in line with how it was documented, this PR fixes the webhook and brings the configuration option in line with the existing auth options

Previously:

  authz:
    method: webhook
    url: https://myauthenticationserver.example.org
    <webhook options>

Now:

  authz:
    method: webhook
    webhook: 
      url: https://myauthenticationserver.example.org
      <webhook options>

I'll merge this in a weeks time unless someone stops me for a review.

Additionally, add a code coverage job to our CI in order to spot some more blind spots like this.

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