Add configuration interface to expose certain config values

[NonSwag]

Previously, people (I) have been using the spigot and paper file configurations to determine whether the server is configured for proxy online mode
Now the spigot interface got deprecated for good reasons, but there is no alternative

This commit introduces methods to check the online mode and enablement status of Velocity and BungeeCord. These changes allow plugins to easily query proxy-related server configurations.

I would like to have some input on the method names since Bukkit uses getOnlineMode
Also, I wasn't really sure on whether to return bungee/velocity online mode true only if the server is actually using bungee or velocity

[Machine-Maker]

I wouldn't be immediately opposed to a new interface where select values from the configuration can be exposed more directly. I wouldn't want even more methods on Bukkit/Server for this though. I think there are probably these and several more values which we can consider stable enough to expose in the API.

[NonSwag]

I wouldn't be immediately opposed to a new interface where select values from the configuration can be exposed more directly. I wouldn't want even more methods on Bukkit/Server for this though. I think there are probably these and several more values which we can consider stable enough to expose in the API.

I like that idea, let me cook something up

[NonSwag]

I've added a new interface ServerConfiguration