Chore: Inbox Members API improvements (#3008)

- New Inbox Member APIs
- Return JSON errors for Platform APIs

swagger/definitions/resource/agent.yml

@@ -0,0 +1,25 @@
+type: object
+properties:
+  id:
+    type: number
+  uid:
+    type: string
+  name:
+    type: string
+  available_name:
+    type: string
+  display_name:
+    type: string
+  email:
+    type: string
+  account_id:
+    type: number
+  role:
+    type: string
+    enum: ['agent', 'administrator']
+  confirmed:
+    type: boolean
+  custom_attributes: 
+    type: object
+    description: Available for users who are created through platform APIs and has custom attributes associated.
+