Add Support for Async Tab Completions for Paper Servers

This adds the ability for plugins to define completion handlers as async safe (not on games main thread)

When they are defined async safe, and ran on a Paper 1.12.2+ server, with a Paper ACF manager,
completions will be handled mostly async, letting you safely do heavier operations in tab completions.

example/src/main/java/co/aikar/acfexample/ACFExample.java

@@ -60,7 +60,7 @@ public final class ACFExample extends JavaPlugin {
                 SomeObject.getContextResolver());
 
         // 4: Register Command Completions - this will be accessible with @CommandCompletion("@test")
-        commandManager.getCommandCompletions().registerCompletion("test", c -> (
+        commandManager.getCommandCompletions().registerAsyncCompletion("test", c -> (
             Lists.newArrayList("foo", "bar", "baz")
         ));