Custom Fields: External ID support (AG-1605)

[moreira]

Problem

Using fieldID to reference a custom field it's hard because it's basically a computer generated UUID (e.g. FieldID-5227880d-0916-4048-a128-3b0930903c5f).
To make the life who is consuming Vendasta's external API easier this PR add supports to an alternative identifier, the External ID.

Solution

Basically the solution is all handled by our internal API's, api-gateway just have to accept the attribute ExternalID as an identifier for custom fields on the Upsert operations.

Upsert example:

{
    "data": {
        "id": "AG-VWD4VW8TGF:ORD-DHRVWDFKWW",
        "type": "orderCustomFields",
        "attributes": {
            "partnerFields": [
                {
                    "externalId": "number-test",
                    "integerValue": 10
                }
            ]
        }
    }
}

List example:

{
    "data": {
        "type": "productCustomFields",
        "id": "VUNI-MP-VFVG5LTKDPPTFV2MRC733PMVPM6DSPB5",
        "attributes": {
            "partnerFields": [
                {
                    "fieldId": "FieldID-8d7ac676-c84e-4afc-bd6a-99a60bde094f",
                    "externalId": "",
                    "title": "Currency test",
                    "description": "Enter currency",
                    "currencyValue": 1000,
                    "fieldType": "currency",
                    "currencyCode": "BRL"
                },
                {
                    "fieldId": "FieldID-dd9746a2-8a6f-465c-9888-a9f0ed9e3763",
                    "externalId": "days-to-use",
                    "title": "Days to use",
                    "description": "Number of days to use the app",
                    "fieldType": "integer"
                }
            ]
        }
    }
}

Notes

I had to remove fieldId's required flag, and I couldn't find a schema that would fieldId or externalId mandatory. There some discussion here, but it looks like it's dead end.

@vendasta/np-easy
@richard-rance
AG-1605

[richard-rance]

We should probably also modify/add a few of the example update requests to show how you can use the new field.