Support mapping of structured metadata fields (from labels to external ids)

[mgriff]

Problem

When importing structured metadata, Cloudinary requires you to use the external_ids for the field and field options (for enum and set fields). This is currently possible for the field names but becomes cumbersome for the field options.

I believe building in functionality to automatically map labels to external ids will be a good addition to the help speed up the migration process

Potential solutions

There are a number of possible solutions (my initial thoughts listed below):

1. Low touch: Pass a metadata object to input2ApiPayload to allow the user to map the external ids (without the need to call the Admin API on each load)
2. Medium touch: Create helper functions to get the external ids, for example getFieldExternalId and getFieldValueExternalId
3. Higher touch: Create a function that takes a JSON object and converts of the metadata (using labels) and converts it to external ids. This object can then be merged with other metadata values input2ApiPayload
4. Highest touch: Somehow we preprocess the row and pass the "initial" upload parameters that the user can then modify in input2ApiPayload. This could review each column in the CSV and try to map it to the expected upload parameters, this could be used for metadata but also Display name, resource type etc.