Add API to easily add metadata to a relationship

[paultranvan]

To add metadata to a relationship, one must typically do this tedious code:

const doc = {
  _id: "mobydick",
  relationships: {
    authors: {
      data: [{
        _id: "hermanmelville",
        _type: "io.cozy.contacts"
      }]
    }
  }
}
doc.relationships.authors.data[0].metadata = { addressId: "123" }
await client.save(doc)

It would be much better with something like doc.relationships.authors.add({_id: ..., metadata: ...})