feat(server): decouple request and response encoding in resource manager

[schoren]

This PR splits the request and response encoding in resource manager. This allows for clients to do requests in one format but expect the response to be in another.

This is useful for example in the CLI, where the client issues a request with a body in YAML format, from the contents of a user created yaml definition file), but then it needs to parse the response to beautify it. Also the user can request the output to be in json.

Instead of having the client manipulate responses, it can just configure the requests with standard http headers.
This reduction in manipulation along the pipeline reduces the need for client code, and reduces the chances of inconsistencies in formats.

Example

Request:

POST /api/exampleresources
Content-Type: text/yaml
Accept: application/json

type: ExampleResource
spec:
  name: the name
  other_value: etc....

Response:

Content-Type: application/json

{
  "type": "ExampleResource",
  "spec": {
    "id": "123",
    "name": "the name",
    "other_value": "etc..."
  }
}

Notes

1. If no relevant header is present, the API fallbacks to JSON, as it always did,
2. Clients that don't need this feature can continue to use just the Content-Type header and the response will fallback to the same content type as the request,
3. Any of Content-Type or Accept header can be used alone, resulting in the previously described behaviour.

Checklist

• tested locally
• added new dependencies
• updated the docs
• added a test