Added option to SharedLibraryFinder interface to specify extraction sub-...

...folder for native libraries.

By default JniGenSharedLibraryLoader extracts each native library to its own temp folder (named by the crc32 of its content). With this small change the application can opt to specify a custom folder name.

I'm working with a setup of two dynamic libraries which depend on each other (libB links to libA), and by extracting them both to the same folder I can avoid the library path issue when libB is loaded.