Chore: Document Conversation APIs in swagger (#552)

* Added the documentation for the conversation APIs - 8 endpoints
* Added descriptions for the existing endpoints
* Added new resource models and updated some parts of the contact API

swagger/paths/conversation/assignments.yml

@@ -0,0 +1,27 @@
+post:
+  tags: [Conversation]
+  summary: Assign Conversation
+  description: Assign a conversation to an agent
+  parameters:
+    - name: id
+      in: path
+      type: number
+      description: id of the conversation
+      required: true
+    - name: data
+      in: body
+      required: true
+      schema:
+        type: object
+        properties:
+          assignee_id:
+            type: number
+  responses:
+    204:
+      description: Success
+      schema:
+        $ref: '#/definitions/user'
+    404:
+      description: Conversation not found
+    403:
+      description: Access denied