add GetEmojiByName method to plugin API (#9641)

plugin/api.go

@@ -200,6 +200,9 @@ type API interface {
 	// GetProfileImage gets user's profile image
 	GetProfileImage(userId string) ([]byte, *model.AppError)
 
+	// GetEmojiByName gets an emoji by it's name.
+	GetEmojiByName(name string) (*model.Emoji, *model.AppError)
+
 	// CopyFileInfos duplicates the FileInfo objects referenced by the given file ids,
 	// recording the given user id as the new creator and returning the new set of file ids.
 	//