feat: add graceful shutdown support, returning UndertowWrapper object

[nikolap]

Resolves #33

Note: this is a breaking change in Java-land, since the object returned is an UndertowWrapper class and not an Undertow class. We sadly cannot proxy Undertow as it is final. An interface or other delegation doesn't work for the same reason.

The option here is to create a wrapper class that exposes all the public methods from Undertow as the same. However, since Undertow is public we still cannot cast to it. As a result, we add a new method getUndertow which returns the original object.

Projects which use this lib and don't check explicitly for the class match should be unaffected so I think it is a low risk breaking change.

[rutchkiwi]

@nikolap wow, you're the best! Thanks.

I've tested this locally now and it works perfectly.

If you want any feedback, it might be nice to write that the default is no timeout in the docs. But it's great!