Add script to pull extism libraries and maven plugin to include them in jar

[Zwiterrion]

Hi, (#3)

I have used this method in other projects. It's really simple but effective. Before bundling the java-sdk, I run a script to fetch and include libextism in the resources folder. In the resources folder, we have every processor architecture.

When the java-sdk jar is included, the right libextism library is loaded.