Restore diplomacy changes and add optional switch to enable/disable

1. In earlier versions of openra, there was optional diplomacy. It was very basic - basically, at any time I can unally or ally with another human player
2. That player receives a notification that I have allied or unallied with them
3. This feature was removed with this change https://github.com/OpenRA/OpenRA/pull/10650
4. I regularly play this game with friends (THANK YOU for creating it, by the way - great to be able to play again), but for us one of the most fun things is to knock out the AI then turn on each other (we all play LAN)
5. So losing 'fragile diplomacy' was very sad, as we're stuck playing the last version that included it
6. This would be to put the option back in just for the Red Alert version, with a switch to enable/disable
7. If the switch is a hidden/command line/config file option, that would be totally fine
8. A nice-to-have would be to add in a notice to all players when an alliance state changes (e.g. Player 1 is now allied with Player 2, The alliance between Player 3 and Player 4 has ended)
9. I did check the bug tracker and I see this issue https://github.com/OpenRA/OpenRA/issues/14910 but I felt that I wanted to add the additional detail above and I didn't want to edit someone else's issue

Hopefully the above makes sense. If it does and it can be tagged appropriately, I would be happy to add a $50 bounty over in bountysource.

Start by reading the removal described in PR #10650 and the related scope in issue #14910. Define the Red Alert-only behavior and configuration switch, then verify that alliance changes produce the requested notifications and that the feature can be enabled or disabled.