- Implemented a proof of concept fix for the incompatibility with Java 15. This incompatibility was caused by the fact that the lambda generated in the NMS.NetworkManager is a hidden class in J15. Starting in Java 15, final fields in hidden classes can no longer be modified regardless of the 'accessible' flag. (see https://openjdk.java.net/jeps/371 "Using a hidden class", point 3). To circumvent this issue, this proof of concept fix retrieves the data from the existing fields in the hidden class (a runnable or a callable) other than the packet. It then retrieves the constructor of the hidden class and instantiates it using the previously-retrieved data and the modified packet instance (this code is only used if the packet instance changed).

- Fixed the patch breaking on Tuinity (and maybe Paper?) because its generated Runnable had a different number of variables, causing an exception when trying to instantiate a new instance with an incorrect number of variables. This update changes it so that it can use any number / order of variables. The variable of the same type as the packet will be replaced by the changed packet (this applies to all variables of this type, but there's only ever 1).

- Introduced a new ObjectReconstructor class that does all the fields/constructor discovering/accessing etc. The Runnable and Callable methods each get one instance of this class so that we can avoid having to get the fields/constructors and set them accessible every time we want to replace a packet.

- Added hardcoded support for Netty's LazyRunnable as used by Tuinity by adding a specific ObjectReconstructor field for it.

- The new ObjectReconstructor method for changing the packet in a schedule packet message is now only used on java 15+. All older versions of Java will use the old method of modifying the packet using reflection, as that is simpler. Inspired by @mikroskeem's efforts towards this PR.

- Switched to using a ConcurrentHashmap for storing the different ObjectReconstructors instead of hardcoding the supported classes.

This fixes the assumption that target class is always a lambda, while it
might not be, thus causing ObjectReconstructor to fail.