feat: add maven check for code signature

[randomicecube]

Closes #12

[randomicecube]

For the JS ecosystem:

Previous info I had

- yarn doesn't seem to allow this yet: https://github.com/yarnpkg/berry/issues/6487 - npm does, with `npm audit signatures`; this requires cloning the repo and installing all dependencies though, would be nice to find an alternative way - this seems promising: https://docs.npmjs.com/about-registry-signatures - pnpm doesn't seem to allow this yet either: https://github.com/pnpm/pnpm/issues/7909

I think the current code should cover everything for the 4 JS-based package managers currently supported? Since they all go through the NPM registry, right?

[randomicecube]

^force push made to remove commit wrongfully made in this branch, cherry-picked to #54