feat: Conversation update API for sla_policy_id (#8973)

- Add an endpoint for updating conversation attributes (priority / sla_policy_id )
- Swagger spec
- minor chores around the conversation API/spec

Fixes: https://linear.app/chatwoot/issue/CW-2100/feat-backend-api-to-update-the-sla-of-a-conversation

swagger/paths/index.yml

@@ -339,6 +339,8 @@
     - $ref: '#/parameters/conversation_id'
   get:
     $ref: ./application/conversation/show.yml
+  patch:
+    $ref: ./application/conversation/update.yml
 /api/v1/accounts/{account_id}/conversations/{conversation_id}/toggle_status:
   parameters:
     - $ref: '#/parameters/account_id'