feat: Exporting all types of Messages so they can be used by consumers

Fixes firebase#1146

Author: BorntraegerMarc

src/messaging/index.ts

@@ -58,15 +58,15 @@ export namespace messaging {
     fcmOptions?: FcmOptions;
   }
 
-  interface TokenMessage extends BaseMessage {
+  export interface TokenMessage extends BaseMessage {
     token: string;
   }
 
-  interface TopicMessage extends BaseMessage {
+  export interface TopicMessage extends BaseMessage {
     topic: string;
   }
 
-  interface ConditionMessage extends BaseMessage {
+  export interface ConditionMessage extends BaseMessage {
     condition: string;
   }