Make Java implementation more idiomatic (by merging my fork)

[ghubstan]

I created a fork with a related PR to see if the biscuit-auth authors might consider inspecting, testing, discussing, and merging it into a new release of the Biscuit Java implementation.

All the changes I made are contained in the PR commits -- best inspected from oldest to newest.

I didn't see a Discussions link, so I created this issue instead. If the biscuit owners want to discuss this privately with me, we'll find a way.