feat: New APIs for search (#6564)

- Adding new API endpoints for search
- Migrations to add appropriate indexes

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

@@ -0,0 +1,14 @@
+json.id message.id
+json.content message.content
+json.message_type message.message_type_before_type_cast
+json.content_type message.content_type
+json.source_id message.source_id
+json.inbox_id message.inbox_id
+json.conversation_id message.conversation.try(:display_id)
+json.created_at message.created_at.to_i
+json.agent do
+  json.partial! 'agent', formats: [:json], agent: message.conversation.try(:assignee) if message.conversation.try(:assignee).present?
+end
+json.inbox do
+  json.partial! 'inbox', formats: [:json], inbox: message.inbox if message.inbox.present? && message.try(:inbox).present?
+end