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/definitions/request/contact/create.yml

@@ -15,3 +15,6 @@ properties:
   identifier: 
     type: string
     description: A unique identifier for the contact in external system
+  custom_attributes:
+    type: object
+    description: An object where you can store custom attributes for contact. example {"type":"customer", "age":30}

swagger/definitions/request/contact/update.yml

@@ -12,3 +12,7 @@ properties:
   identifier: 
     type: string
     description: A unique identifier for the contact in external system
+  custom_attributes:
+    type: object
+    description: An object where you can store custom attributes for contact. example {"type":"customer", "age":30}
+