Add External ID to MigrationReference

[nkooman-bzs]

Motivation

Why is this feature required? What problems does it solve?

When importing from a 3rd-party system I want to be able to reference linked items and assets by external ID explicitly.

Proposed solution

Expand the MigrationReference type to accept a discriminated union of { readonly codename: T; } | { readonly external_id: T; }.
Update content item and asset importers to handle external IDs.

Additional context

N/A

[Enngage]

We'll explore this functionality in upcoming quarter to see if it's something we want to implement here ;)