Chore: API Improvements (#2956)

- API to fetch info of a single inbox
- Document passing custom_attributes in the API
- Ability to filter contacts with contact identifier in search API

spec/services/contacts/contactable_inboxes_service_spec.rb

@@ -2,7 +2,7 @@ require 'rails_helper'
 
 describe Contacts::ContactableInboxesService do
   let(:account) { create(:account) }
-  let(:contact) { create(:contact, account: account) }
+  let(:contact) { create(:contact, account: account, email: 'contact@example.com', phone_number: '+2320000') }
   let!(:twilio_sms) { create(:channel_twilio_sms, account: account) }
   let!(:twilio_sms_inbox) { create(:inbox, channel: twilio_sms, account: account) }
   let!(:twilio_whatsapp) { create(:channel_twilio_sms, medium: :whatsapp, account: account) }