[FEATURE ds-payload-hooks] Add hooks to map type in payload to modelName

[pangratz]

The modelNameFromPayloadKey and payloadKeyFromModelName hooks on the
serializer allow to customize the mapping between the key of a JSON and
the corresponding name of the model. Consider the following payload:

{
  "blog/post": {
    "id": 1
  }
}

To map the "blog/post" key to the post model, the
modelNameFromPayloadKey hook is used:

serializer.modelNameFromPayloadKey("blog/post")

---

There are some issues with the current code base, as the
modelNameFromPayloadKey and payloadKeyFromModelName hooks are also
used to map the "value" of a type from / to the payload. Consider the
following payload of a JSON-API document:

{
  "data": {
    "id": 1,
    "type": "API:V1::User"
  }
}

To map the namespaced type to the model name ember-data is using,
currently the modelNameFromPayloadKey is used:

serializer.modelNameFromPayloadKey("API::V1::User")

Now this gets complicated if your API responds with the following
payload (using custom key and custom types for polymorphic records for
example):

{
  "blog/post": {
    "id": 1,
    "user": 2,
    "userType": "API:V1::Administrator"
  }
}

Now the modelNameFromPayloadKey is invoked for both:

serializer.modelNameFromPayloadKey("blog/post")
serializer.modelNameFromPayloadKey("API::V1::Administrator")

This means that the logic within the hook would get complicated.

---

As the name suggests, the method should only be used to map the key, not
the value. This commit adds modelNameFromPayloadType and
payloadTypeFromModelName hooks and uses them during normalization and
serialization.

This commit maintains the old behavior using, if it is used and logs a
deprecation warning to move to the new hooks.

---

This PR should help tackling the issues raised in #3801 and #4208.

[bmac]

Overall looks good. Do you mind wrapping this in a feature flag?

[bmac]

It seems like if a user intentionally wanted modelNameFromPayloadType and modelNameFromPayloadKey to return different values and they needed to define a custom modelNameFromPayloadType we would always hit the deprecated case.

What about if we always called modelNameFromPayloadType but had its default implementation just call modelNameFromPayloadKey?

[fivetanley]

We should also make an ember-watson transform after this gets merged.

[pangratz]

@bmac I've updated the code and put everything behind the ds-payload-type-hooks feature flag.

You're comment made me re-think the approach and now the following "rules" apply:

• modelNameFromPayloadKey and payloadKeyFromModelName are deprecated on json and json-api serializer, since the their payload doesn't have the type in any key; there is a deprecation warning to use the new modelNameFromPayloadType and payloadTypeFromModelName hooks instead
• rest serializer supports all 4 hooks (payloadKey and payloadType) since they make sense for that payload
  • if the value for the new hook differs with the old one, then a deprecation is logged iff the new hook is not implemented but the old one is
  • if they are the same then everything is ok