Vanilla Java client

[reva]

PR #47 was never merged. I took another stab at it, using only vanilla java. For a server, some library would be useful, but there are too many options. This should give a good starting point, because it at least shows how to mangle the key stores. In any case I would say that most of the time you have a reverse proxy that terminates your HTTPs and mTLS in front of your java server anyway.

I tried to document which version of yarn and npm to use for the build, hope this makes sense.

[CLAassistant]

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