Improve security of the solution via e.g. MD5 checksums

[matiwinnetou]

Allowing native code to run on anybody's machine is dangerous.

1. Currently download_libs.sh doesn't contain md5 checksums. IMHO this is necessary.
2. Ideally a developer should never add those to native folder but actually only it should be done via CI upon verification of md5 checksums

There could be other measures / ideas taken for this but the things above should be minimum, especially that project is from cryptocurrency / where trojan horses stealing things like wallet passwords are common.

[matiwinnetou]

Alternatives to discuss / think about:

• Find a way to import cargo crates into gradle project
• Merge those two projects together: aiken-jna-wrapper and aiken-java-binding and invoke cargo via gradle tasks.