Lua API: introduce effects.unit_bonus()
This, combined with #41745, gives Lua access to the unit owner's diplomatic relationships - including local relationships - and to unit class flags as long as the ruleset author is willing to spend a user effect.
This, combined with #41745, gives Lua access to the unit owner's diplomatic relationships - including local relationships - and to unit class flags as long as the ruleset author is willing to spend a user effect.