Add cargo goal for specific type of cargo

Create an objective/goal for delivering a specific cargo, not just any cargo (requires tracking cargo deliveries manually)

The issue does not name any files, tests, or entry points. Start by locating the existing objective and cargo-delivery handling in the OpenTTD game script, then determine how a specific cargo type can be tracked and verified. Done means a goal can target one cargo type and recognize its deliveries.

Written by the indexing model from the issue text.