Added service register and unregister events. Addresses BUKKIT-816

By: zml2008 <zach@zachsthings.com>

paper-api/src/main/java/org/bukkit/event/server/ServiceEvent.java

@@ -0,0 +1,18 @@
+package org.bukkit.event.server;
+
+import org.bukkit.plugin.RegisteredServiceProvider;
+
+/**
+ * An event relating to a registered service. This is called in a {@link org.bukkit.plugin.ServicesManager}
+ */
+public abstract class ServiceEvent extends ServerEvent {
+    private final RegisteredServiceProvider<?> provider;
+
+    public ServiceEvent(final RegisteredServiceProvider<?> provider) {
+        this.provider = provider;
+    }
+
+    public RegisteredServiceProvider<?> getProvider() {
+        return provider;
+    }
+}

paper-api/src/main/java/org/bukkit/event/server/ServiceRegisterEvent.java

@@ -0,0 +1,23 @@
+package org.bukkit.event.server;
+
+import org.bukkit.event.HandlerList;
+import org.bukkit.plugin.RegisteredServiceProvider;
+
+/**
+ * This event is called when a service is registered.
+ */
+public class ServiceRegisterEvent extends ServiceEvent {
+    private static final HandlerList handlers = new HandlerList();
+
+    public ServiceRegisterEvent(RegisteredServiceProvider<?> registeredProvider) {
+        super(registeredProvider);
+    }
+
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+}

paper-api/src/main/java/org/bukkit/event/server/ServiceUnregisterEvent.java

@@ -0,0 +1,23 @@
+package org.bukkit.event.server;
+
+import org.bukkit.event.HandlerList;
+import org.bukkit.plugin.RegisteredServiceProvider;
+
+/**
+ * This event is called when a service is unregistered.
+ */
+public class ServiceUnregisterEvent extends ServiceEvent {
+    private static final HandlerList handlers = new HandlerList();
+
+    public ServiceUnregisterEvent(RegisteredServiceProvider<?> serviceProvider) {
+        super(serviceProvider);
+    }
+
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+}