feat: add ProxyPreShutdownEvent before players are disconnected (#1626)

* feat: delay player disconnect until ProxyShutdownEvent completes

* fix: added back empty line

* feat: added ProxyPreShutdownEvent

* feat: CR changes

* chore: fixed license, annotated with Beta annotation

* Update proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

Co-authored-by: Timon Seidel <timong.seidel@gmail.com>

* Update proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

Co-authored-by: Timon Seidel <timong.seidel@gmail.com>

* chore: consolidated log message

* Update proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

Co-authored-by: Timon Seidel <timong.seidel@gmail.com>

* Update proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

Co-authored-by: powercas_gamer <cas@mizule.dev>

* Update api/src/main/java/com/velocitypowered/api/event/proxy/ProxyPreShutdownEvent.java

Co-authored-by: powercas_gamer <cas@mizule.dev>

* feat: make ProxyPreShutdownEvent timeout configurable via system property

* fix: cs

* Document velocity.pre-shutdown-timeout system property

---------

Co-authored-by: Timon Seidel <timong.seidel@gmail.com>
Co-authored-by: powercas_gamer <cas@mizule.dev>
Co-authored-by: Adrian Gonzales <adriangonzalesval@gmail.com>

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

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2018-2025 Velocity Contributors
+ *
+ * The Velocity API is licensed under the terms of the MIT License. For more details,
+ * reference the LICENSE file in the api top-level directory.
+ */
+
+package com.velocitypowered.api.event.proxy;
+
+import com.google.common.annotations.Beta;
+import com.velocitypowered.api.event.annotation.AwaitingEvent;
+
+/**
+ * This event is fired by the proxy after it has stopped accepting new connections,
+ * but before players are disconnected.
+ * This is the last point at which you can interact with currently connected players,
+ * for example to transfer them to another proxy or perform other cleanup tasks.
+ *
+ * @implNote Velocity will wait for all event listeners to complete before disconnecting players,
+ *     but note that the event will time out after the configured value of the
+ *     <code>velocity.pre-shutdown-timeout</code> system property, default 10 seconds,
+ *     in seconds to prevent shutdown from hanging indefinitely
+ * @since 3.4.0
+ */
+@Beta
+@AwaitingEvent
+public final class ProxyPreShutdownEvent {
+
+  @Override
+  public String toString() {
+    return "ProxyPreShutdownEvent";
+  }
+}