feat: acceptGroupV4Invite (#677)

* Added option client.acceptGroupV4Invite()

* Update Client.js

* Update index.d.ts

Added a description to the inviteV4 object

Renamed "invite" by "inviteV4"

* Update Client.js

* Update Message.js

renamed "invite" to "inviteV4"

* Update Client.js

* Added message.acceptGroupV4Invite method

* Update index.d.ts

acceptGroupV4Invite

* better typings, refactor some names

Co-authored-by: Pedro Lopez <pedroslopez@me.com>

src/util/Constants.js

@@ -72,7 +72,8 @@ exports.MessageTypes = {
     ORDER: 'order',
     REVOKED: 'revoked',
     PRODUCT: 'product',
-    UNKNOWN: 'unknown'
+    UNKNOWN: 'unknown',
+    GROUP_INVITE: 'groups_v4_invite'
 };
 
 /**