Add unsupported --legacy flag for targeting Unreal Engine 4

[hach-que]

Fab added support for Unreal Engine code plugins to submit updates for engine versions older than the last 3 versions, at least going back to 4.27.

This was my initial attempt to add 4.x support to UET via a --legacy flag, however in practice:

• My build environment isn't set up for UE4 any more and UET can't auto-install dependencies for that engine version, and
• Most of my own plugin code has dropped the relevant code guards necessary to support UE4 (even for plugins like OSB or RSI).

Since I can't actually test everything works end-to-end with this flag, I don't intend to merge it in at this time.