Initial reload command support.

What is missing:
- Changing/removing servers "in-flight"
- Query stuff
- A way to preserve existing user connections while closing the old port

api/src/main/java/com/velocitypowered/api/event/proxy/ProxyReloadEvent.java

@@ -0,0 +1,12 @@
+package com.velocitypowered.api.event.proxy;
+
+/**
+ * This event is fired when the proxy is reloaded by the user using {@code /velocity reload}.
+ */
+public class ProxyReloadEvent {
+
+  @Override
+  public String toString() {
+    return "ProxyInitializeEvent";
+  }
+}