feat: Conversation and contact endpoint (#3198)

app/views/api/v1/models/_conversation.json.jbuilder

@@ -0,0 +1,21 @@
+json.id conversation.display_id
+json.created_at conversation.created_at.to_i
+json.contact do
+  json.id conversation.contact.id
+  json.name conversation.contact.name
+end
+json.inbox do
+  json.id conversation.inbox.id
+  json.name conversation.inbox.name
+  json.channel_type conversation.inbox.channel_type
+end
+json.messages do
+  json.array! conversation.messages do |message|
+    json.content message.content
+    json.id message.id
+    json.sender_name message.sender.name if message.sender
+    json.message_type message.message_type_before_type_cast
+    json.created_at message.created_at.to_i
+  end
+end
+json.account_id conversation.account_id