The extraMetadata field was introduced in order to store custom metadata without modifying the model.
After the introduction of the new metadata json schema and the possibility to access the sparse metadata fields, the extraMetadata field becomes outdated.

External clients (if any) using the API for handling the extraMetadata should be updated to use the new sparse field API.

So, at least, we need:

• migration to move extraMetadata content into sparse fields
• extraMetadatata field removal + related migration

IF we want to maintain back compatibility, we need to make sure that read and write accesses for the extraMetadata still work:

• refact the views that handle extraMetadata field as input, and store the inner properties in SparseFields
• refact the serializers outputting the extraMetadata, making them read the sparsefields instead.

@giohappy These last two points needs discussion to find out if they are really needed.