As a user of Quay, I should be able to register new OCI artifact types through the config-tool

We define a set of default OCI artifact types in config.py. This means that if additional types needs to be registered on top of the default ones, one would need to manually copy the default value of ALLOWED_OCI_ARTIFACT_TYPES from config.py and add to that, which is not a great UX.

Ideally, the config-tool would have some form pre-filled with the default values, and allow new types to be added, and generated/validated as part of the config bundle.