Add way to set a world seed for a multiplayer server

Adding a way to set a seed for a multiplayer server would be EXTREMELY useful for me as I essentially only play on servers where i have the seed. I could also see maybe also using data from [servux](https://github.com/sakura-ryoko/servux/blob/9a41006d4d3ac8034fc52365afc0296e42c668ce/src/main/java/fi/dy/masa/servux/dataproviders/HudDataProvider.java#L411)? Not sure if that would be viable though