Improve the castle reinforcement logic for AI (implement the long-standing TODO comment)

[oleg-derevenetz]

• The logic of castle reinforcement has been streamlined in the sense that just one AI-only method has been made instead of several different methods scattered across different classes (the same or better result is achieved in fewer actions);
• AI is now able to hire non-upgraded units if it does not have enough money for upgraded ones, just like a human player is able to do this in fheroes2 (but not in the original game!) - for instance, if AI does not have enough gold or sulfur for a Black Dragon, it can hire a Red one;
• AI now tries to make room for new units by upgrading and combining stacks of existing units (this logic was present before, but was more chaotic and irregular);
• AI is now able to merge same units from garrison to the guest hero's army if necessary to make room for new units.

[github-actions]

⚠️ Clang-Tidy found issue(s) with the introduced code (1/1)