feat: Ability to search conversation with message content (#1265)

- API end point which takes message content as search parameter
- end point supports additional filtering with labels and inbox ids
- swagger doc

app/views/api/v1/accounts/conversations/search.json.jbuilder

@@ -0,0 +1,22 @@
+json.data do
+  json.meta do
+    json.mine_count @conversations_count[:mine_count]
+    json.unassigned_count @conversations_count[:unassigned_count]
+    json.all_count @conversations_count[:all_count]
+  end
+  json.payload do
+    json.array! @conversations do |conversation|
+      json.inbox_id conversation.inbox_id
+      json.messages conversation.messages
+      json.status conversation.status
+      json.muted conversation.muted?
+      json.can_reply conversation.can_reply?
+      json.timestamp conversation.messages.last.try(:created_at).try(:to_i)
+      json.contact_last_seen_at conversation.contact_last_seen_at.to_i
+      json.agent_last_seen_at conversation.agent_last_seen_at.to_i
+      json.additional_attributes conversation.additional_attributes
+      json.account_id conversation.account_id
+      json.labels conversation.label_list
+    end
+  end
+end