[MM-16709] Add helper method for plugins using MessageHasBeenPosted (#12539)

The PR introduces a method in helper interface. The method has common code for filtering to be used across plugins which use MessageHasBeenPosted.

plugin/helpers.go

@@ -57,6 +57,18 @@ type Helpers interface {
 	//
 	// Minimum server version: 5.6
 	KVSetWithExpiryJSON(key string, value interface{}, expireInSeconds int64) error
+
+	// ShouldProcessMessage returns if the message should be processed by a message hook.
+	//
+	// Use this method to avoid processing unnecessary messages in a MessageHasBeenPosted
+	// or MessageWillBePosted hook, and indeed in some cases avoid an infinite loop between
+	// two automated bots or plugins.
+	//
+	// The behaviour is customizable using the given options, since plugin needs may vary.
+	// By default, system messages and messages from bots will be skipped.
+	//
+	// Minimum server version: 5.2
+	ShouldProcessMessage(post *model.Post, options ...ShouldProcessMessageOption) (bool, error)
 }
 
 // HelpersImpl implements the helpers interface with an API that retrieves data on behalf of the plugin.