Support using multiple different network plugins (#1151)

## Type of Change
- [x] New feature  
- [x] Breaking change

## Motivation and Context
We want to be able to support using multiple network plugins during
`container`'s lifetime. This additionally means needing to pick an
interface strategy to interpret a network attachment based on what
network plugin was used to create that attachment. This PR will
potentially replace https://github.com/apple/container/pull/1081.

Followups:
- doc updates to include the ability to specify plugin in the network
creation cli

## Testing
- [x] Tested locally
- [x] Added/updated tests

Sources/ContainerXPC/XPCMessage.swift

@@ -269,6 +269,12 @@ extension XPCMessage {
         }
     }
 
+    public func set(key: String, xpcDictionary: xpc_object_t) {
+        lock.withLock {
+            xpc_dictionary_set_value(self.object, key, xpcDictionary)
+        }
+    }
+
     public func endpoint(key: String) -> xpc_endpoint_t? {
         lock.withLock {
             xpc_dictionary_get_value(self.object, key)