As a Korifi API user I want to be able to get service plan fields when listing service instances

[danail-branekov]

GIVEN I have enabled the experimental managed services flag in the helm values
WHEN I have created service brokers
WHEN I /v3/service_instances?fields[service_plan]=guid,name,relationships.service_offering
THEN I see the guids, names and related offering guids under included.service_plans array, for example

{
  "included": {
    "service_plans": [
      {
        "guid": "aaa3d392-f8f9-504f-a216-a579cbc24159",
        "name": "application",
        "relationships": {
          "service_offering": {
            "data": {
              "guid": "fc018c50-eadc-57a8-b231-fbcf147eaf78"
            }
          }
        }
      }
    ]
  },
  "pagination": {
  ...
  },
  "resources": [
    {
      "guid": "a07c8a84-1e85-4882-9764-fb85f57f46bd",
      "name": "my-service-instance",
      ...
  ]
}

Dev notes:

• Managed services proposal
• List service instances docs
• Fields concept docs