feat: Introduce last responding agent option to automation assign agent (#12326)

Introduce a `Last Responding Agent` options to assign_agents action in
automations to cover the following use cases.

- Assign conversations to first responding agent : ( automation message
created at , if assignee is nil, assign last responding agent )
- Ensure conversations are not resolved with out an assignee : (
automation conversation resolved at : if assignee is nil, assign last
responding agent )

and potential other cases.

fixes: #1592

app/javascript/dashboard/i18n/locale/en/automation.json

@@ -130,6 +130,7 @@
       "ATLEAST_ONE_ACTION_REQUIRED": "At least one action is required"
     },
     "NONE_OPTION": "None",
+    "LAST_RESPONDING_AGENT": "Last Responding Agent",
     "EVENTS": {
       "CONVERSATION_CREATED": "Conversation Created",
       "CONVERSATION_UPDATED": "Conversation Updated",