Chore: API Improvements (#2956)

- API to fetch info of a single inbox
- Document passing custom_attributes in the API
- Ability to filter contacts with contact identifier in search API

swagger/paths/inboxes/index.yml

@@ -4,6 +4,8 @@ get:
   operationId: listAllInboxes
   summary: List all inboxes
   description: List all inboxes available in the current account
+  parameters:
+    - $ref: '#/parameters/account_id'
   responses:
     200:
       description: Success