[RFD322] Resource name selectors within request bodies

Some endpoints use resource name selectors within the request bodies like NetworkInterfaceCreate:

      "NetworkInterfaceCreate": {
        "description": "Create-time parameters for a [`NetworkInterface`](omicron_common::api::external::NetworkInterface)",
        "type": "object",
        "properties": {
          "description": {
            "type": "string"
          },
          "ip": {
            "nullable": true,
            "description": "The IP address for the interface. One will be auto-assigned if not provided.",
            "type": "string",
            "format": "ip"
          },
          "name": {
            "$ref": "#/components/schemas/Name"
          },
          "subnet_name": {
            "description": "The VPC Subnet in which to create the interface.",
            "allOf": [
              {
                "$ref": "#/components/schemas/Name"
              }
            ]
          },
          "vpc_name": {
            "description": "The VPC in which to create the interface.",
            "allOf": [
              {
                "$ref": "#/components/schemas/Name"
              }
            ]
          }
        },
        "required": [
          "description",
          "name",
          "subnet_name",
          "vpc_name"
        ]
      },

These request bodies create the same problems as using resource name identifiers in the paths. We should come up with a solution for these bodies as well. As we can't have queries within the request body JSON payload, it probably makes sense to use only the direct parent's UUID for these cases.

Open to discussing this if anyone has a different opinion though!

cc @ahl @zephraph @david-crespo

Review the NetworkInterfaceCreate schema and the other request bodies that use resource name selectors. First understand how these selectors create the same problems as resource names in paths, then document the agreed solution for identifying direct parents and define what updated API schemas and behavior would demonstrate that the issue is resolved.