Create ID for embedded collections

If a there is a `hasMany` relationship to another domain class, and that collection is embedded, provide a configuration option to tell GORM to assign an id to items as they are added in the collection.

Start by locating GORM handling for hasMany relationships and embedded collections, then trace how collection items currently receive identifiers. Define the configuration option so IDs are assigned when items are added, and add or update tests covering embedded collection items and the opt-in behavior.

Written by the indexing model from the issue text.