2.0: Protobuf / gRPC: how to pass network configuration to plugins?

[squeed]

Assuming we move forward with daemonization (#821), then we will have to write a protobuf schema for the CNI protocol. This tpresents a few awkward edges around network configuration.

The gist of the issue is this:

1. A "network" and a "configuration" are the same thing
2. The same plugin can be used by multiple "networks"
   2.1. The same plugin can be used in different networks in the same container.
3. A network configuration contains arbitrary, plugin-specific rich data.

So, we need some way to ergonomically pass, to a plugin, a network configuration. This configuration is user-defined but plugin specific.

Options

1. Embed json in protobuf
2. The protobuf any type, which supports embedding arbitrary protobuf messages.
3. ???

[robberphex]

In CNI 2.0, we provide gRPC/Protobuf definition, so I think use protobuf any type is nature.

Example network configuration could be like:

{
  "cniVersion": "1.0.0",
  "name": "dbnet",
  "plugins": [
    {
      "type": "bridge",
      "bridge": "cni0",
      "extra": [
        {
          // **plugin specific parameters for nerdctl**
          "@type": "type.googleapis.com/helloworld.NerdctlNetworkConfig",
          "NerdctlID": 112233,
          "NerdctlLabels": {
            "bob": "11"
          }
        },
        {
          // **plugin specific parameters for bridge**
          "@type": "type.googleapis.com/k8s.cni.cncf.io/v1.0.0.BridgeNetworkConfig",
          "configKey": "configValue"
        }
      ],
      "ipam": {
        "type": "host-local",
        // ipam specific
        "subnet": "10.1.0.0/16",
        "gateway": "10.1.0.1",
        "routes": [
          {
            "dst": "0.0.0.0/0"
          }
        ]
      },
      "dns": {
        "nameservers": [
          "10.1.0.1"
        ]
      }
    },
    {
      "type": "tuning",
      "capabilities": {
        "mac": true
      },
      "sysctl": {
        "net.core.somaxconn": "500"
      }
    },
    {
      "type": "portmap",
      "capabilities": {
        "portMappings": true
      }
    }
  ]
}

And protobuf any type can be converted to json.

I submitted a pr, #874 .