refactor(host_network): extract NetworkManager as a reusable component (#183)

The NetworkManager logic was implemented directly into the `HaClusterTopology`, which wasn't directly its concern and prevented us from being able to reuse that NetworkManaager implementations in the future for a different Topology.

* Extract a `NetworkManager` trait
* Implement a `OpenShiftNmStateNetworkManager` for `NetworkManager`
* Dynamically instantiate the NetworkManager in the Topology to delegate calls to it

Reviewed-on: #183
Reviewed-by: johnride <jg@nationtech.io>

examples/okd_pxe/src/topology.rs

@@ -10,7 +10,10 @@ use harmony::{
 use harmony_macros::{ip, ipv4};
 use harmony_secret::{Secret, SecretManager};
 use serde::{Deserialize, Serialize};
-use std::{net::IpAddr, sync::Arc};
+use std::{
+    net::IpAddr,
+    sync::{Arc, OnceLock},
+};
 
 pub async fn get_topology() -> HAClusterTopology {
     let firewall = harmony::topology::LogicalHost {
@@ -76,6 +79,7 @@ pub async fn get_topology() -> HAClusterTopology {
         },
         workers: vec![],
         switch_client: switch_client.clone(),
+        network_manager: OnceLock::new(),
     }
 }