Add external ID to Minder entities

[dmjb]

Please describe the enhancement

Per recent discussion on providers, add an external ID to artifacts and repositories. This will be an opaque identifier which the providers will use to identify the entity in the third party API.

Solution Proposal

Per discussion with Evan - store the ID information in a protobuf, serialize, and store in the DB.

Describe alternatives you've considered

No response

Additional context

No response

Acceptance Criteria

No response