Added GETEmojilist plugin api (#9750)

* Add GetEmojiList plugin api

* Fixed bug in getemojilist causing build test failure

* Fix linting error

* Add requested changes

* Fix all conflicts

plugin/api.go

@@ -259,6 +259,13 @@ type API interface {
 	// Minimum server version: 5.6
 	GetProfileImage(userId string) ([]byte, *model.AppError)
 
+	// GetEmojiList returns a page of custom emoji on the system.
+	//
+	// The sortBy parameter can be: "name".
+	//
+	// Minimum server version: 5.6
+	GetEmojiList(sortBy string, page, perPage int) ([]*model.Emoji, *model.AppError)
+
 	// GetEmojiByName gets an emoji by it's name.
 	//
 	// Minimum server version: 5.6