Fix illegal reflective access operation has occurred (Java 9+)

[SnowyCoder]

The ExternalJarUtil, used to load database drivers (and maybe script engines) needs to make an illegal reflection to UrlClassLoader#addURL function, we need to find a workaround for this before the illegal reflections are denied

[SnowyCoder]

Here's an idea for a possible solution: Add the driver adaptors in an external Jar (maybe using the Service system or something similar) then load them using a custom ClassLoader that extends the URLClassLoader class and redefines addURL (it's protected).
This would let us manage our own ClassLoader, loading the driver JARs without any reflection at all.
Still this requires adding a gradle-module for this pretty stupid spigot issue.