feat: Custom Attributes for contacts (#1158)

Co-authored-by: Pranav Raj Sreepuram <pranavrajs@gmail.com>

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

@@ -5,6 +5,7 @@ json.id resource.id
 json.name resource.name
 json.phone_number resource.phone_number
 json.thumbnail resource.avatar_url
+json.custom_attributes resource.custom_attributes
 
 # we only want to output contact inbox when its /contacts endpoints
 if defined?(with_contact_inboxes) && with_contact_inboxes.present?