Allow running Velocity without any servers.

This is a niche setup, however if your network is 100% dynamically configured, this is a handy feature to have available.

To support this functionality, a new PlayerChooseInitialServerEvent event was added to allow the initial server to connect to be changed as desired.

api/src/main/java/com/velocitypowered/api/event/player/PlayerChooseInitialServerEvent.java

@@ -0,0 +1,51 @@
+package com.velocitypowered.api.event.player;
+
+import com.google.common.base.Preconditions;
+import com.velocitypowered.api.proxy.Player;
+import com.velocitypowered.api.proxy.server.RegisteredServer;
+import java.util.Optional;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+/**
+ * Fired when a player has finished connecting to the proxy and we need to choose the first server
+ * to connect to.
+ */
+public class PlayerChooseInitialServerEvent {
+
+  private final Player player;
+  private @Nullable RegisteredServer initialServer;
+
+  /**
+   * Constructs a PlayerChooseInitialServerEvent.
+   * @param player the player that was connected
+   * @param initialServer the initial server selected, may be {@code null}
+   */
+  public PlayerChooseInitialServerEvent(Player player, @Nullable RegisteredServer initialServer) {
+    this.player = Preconditions.checkNotNull(player, "player");
+    this.initialServer = initialServer;
+  }
+
+  public Player getPlayer() {
+    return player;
+  }
+
+  public Optional<RegisteredServer> getInitialServer() {
+    return Optional.ofNullable(initialServer);
+  }
+
+  /**
+   * Sets the new initial server.
+   * @param server the initial server the player should connect to
+   */
+  public void setInitialServer(RegisteredServer server) {
+    this.initialServer = server;
+  }
+
+  @Override
+  public String toString() {
+    return "PlayerChooseInitialServerEvent{"
+        + "player=" + player
+        + ", initialServer=" + initialServer
+        + '}';
+  }
+}