Add support for nested entitlements relationship during policy creation

[ezekg]

Similar to what we already do for releases, let's allow policies to be created with entitlements already attached.

POST /v1/policies
{
  "data": {
    "type": "policy",
    "attributes": {
      "name": "Feature Policy"
    },
    "relationships": {
      "entitlements": {
        "data": [
          { "type": "entitlement", "id": "fc39bf6f-5232-40d3-8980-7f9c109e9e98" },
          { "type": "entitlement", "id": "82a9c852-b864-4c8a-9e51-9bfe53906f5f" },
          { "type": "entitlement", "attributes": { "code": "NEW_FEATURE" } }
        ]
      }
    }
  }
}

Save a request or two.

[ezekg]

May also be nice to add support for the same during license creation, but that may be more complex i.r.t. authz.