Add support for Open Source Registry endpoints

[josias-brenner]

This PR add support for endpoints to receive webhook notifications from the Open Source Registry (see issue).

The endpoint can be used in the following way:

# Kobold config
endpoints:
  - name: open-source-registry
    type: osr
    path: /osr/example-endpoint

# Registry config
version: 0.1
http:
  addr: 0.0.0.0:5000
storage:
  filesystem:
    rootdirectory: /var/lib/registry
notifications:
  events:
    includereferences: false
  endpoints:
    - name: kobold
      disabled: false
      url: https://url-to-kobold-instance:8000/osr/example-endpoint
      timeout: 1s
      threshold: 10
      backoff: 5s
      ignore:
        mediatypes:
          - application/octet-stream
        actions:
          - pull
          - mount
          - delete

I'm still not certain what this registry-implementation is called. The repository on docker-hub ist just registry, the documentation was recently moved to https://distribution.github.io/distribution/ and the README.md off it's source code (also at https://github.com/distribution/distribution/) says: "This repository's main product is the Open Source Registry".

[bluebrown]

Nice, thanks for the PR!. Maybe the type could be called distribution ? I was also thinking about this yesterday.

[josias-brenner]

Nice, thanks for the PR!. Maybe the type could be called distribution ? I was also thinking about this yesterday.

I like distribution. I'll adjust the PR.