optional dependencies (don't block startup)

For example, Dogenet tracks other dogenet nodes but if it has access to Core it can track the nodes Core caches, this is a good example of an optional dependency. 

```json
  "dependencies": [
    {
      "interfaceName": "core-rpc",
      "interfaceVersion": "0.0.1",
      "optional": true,      <<<<<<<<<<<<< proposed change
      "permissionGroups": [
        "RPC"
      ],
    },

```