Add support for PvE startboxes

[p2004a]

Relevant threads on Discord:

• Explanation for Yarbiz: https://discord.com/channels/549281623154229250/1127206311893930105/1127879322569560074
• Explanation from Damgam: https://discord.com/channels/549281623154229250/554307356360441876/1149457851870351513

Need to

• Test in local Chobby checkout the extraBox format, and if it does what expected for the vs AI and Raptors/Scav setup.
• Extend Rowy strtbox datatype for with extraBox.
• Extend Rowy UI with extraBox/"PvE" toggle: we need to support only extraBox=1 atm.
• Adjust startboxes generation code to output the extraBox.

Nice to have:

• When startbox for PvE is not defined, copy team box as PvE default.

[burnhamrobertp]

Only certain portions of this work will be done in this codebase, but before anyone can do this work, it seems as though we'd need to verify the first bullet point above ^

Test in local Chobby checkout the extraBox format, and if it does what expected for the vs AI and Raptors/Scav setup.

Provided it works as expected given a particular format of the extraBox data, modifications to the scripts for maps-metadata can be made to add extraBox when appropriate.