Merge pull request #504 from Catnies/dev-contextholder

contextholder添加get方法

core/src/main/java/net/momirealms/craftengine/core/plugin/context/ContextHolder.java

@@ -63,6 +63,11 @@ public class ContextHolder {
         return this;
     }
 
+    @SuppressWarnings("unchecked")
+    public <T> T getOrNull(ContextKey<T> parameter) {
+        return (T) Optional.ofNullable(this.params.get(parameter)).map(Supplier::get).orElse(null);
+    }
+
     @SuppressWarnings("unchecked")
     public <T> T getOrThrow(ContextKey<T> parameter) {
         Supplier<T> object = (Supplier<T>) this.params.get(parameter);