Add MonsterSpawn_Group random map spawn

Hello developers,

Through this message, I would like to suggest adding the possibility of creating invasions with the option to define random maps.

This will make the invasions more elaborate.

Currently, to do this, we have to add the same invasion multiple times, reducing the success rate of appearances, but this is not efficient.

With this feature, we could add more than one map in just one invasion, something like this:

<Map Number="0;2;3" RandomMapEnable="1" / >

If enabled, the map will be randomly selected by the system; if set to 0, the invasion would happen on all three maps simultaneously.