Add API for querying custom groups

Use case: the API consumer wants to create a form for a new contact, so they want to know what custom data groups/fields extend a new contact (or a new individual, or a new parent - or a new event for that matter).

They would like to make a call like civicrm_custom_group_search() with the right params and get back a hierarchy of custom data groups/fields with their types and information what's required.