Importing relationships via CSV

[FelixMalfait]

The biggest limitation of our CSV feature today is that it's not possible to use it to create relationships. So for example if you import a list of people, it can't automatically create/link companies attached to it.

There are different ways to approach this problem. I think the best way for now would be to allow people to include unique identifiers that would automatically be surfaced. A unique identified can be: the id, and external if (e.g. SalesforceId), but also sometimes a business key (email for persons or domain for company).

Unfortunately we don't really have a good way to add unicity constraint to a field, and therefore fetch fields that could be good candidates in a good way. So for v1 I suggest we retried id, externalId only for all objects. And hardcode domain/email for company/people.

So on the frontend, for a given object, we need to add to the list of matchable fields all the fields the one that are coming from relations ([id, externalId] in most cases, and that +domain/email for company/person). And on the backend side, we need to adapt our resolvers to also be able to match through this (company.domain, siblingObject.externalId, etc.)