server.properties generator-settings for level-type FLAT not implemented; property is stored in ignored flat_world_options NBT

The bug

Generation of custom world only using the server.properties on a multi-player server for a FLAT level-type is not possible.

When level-type is FLAT, the generator-settings is parsed into the flat_world_options json property that afaik is never used.

Should be either parsed into this flat_world_options and code should be implemented to convert "preset" data into the appropriate json format that is being put into the generator-settings key of the level.dat or allow using the new json format in generator-settings similar to BUFFET.

Note, that when level-type is BUFFET, the generator-settings take a json string that is directly used for the generator-settings key in the level.dat, so it works as expected.

But for FLAT, it doesn't implements json in this new manner, and doesn't look like it is parsing the flat_world_options as the "preset" or legacy format.  Documentation is limited.