Box.V2.Samples.JWTAuth - better day 1 experience

Currently if you clone the project and build you will get a build error about a missing `private_key.pem`, and you'll have to go look for that file and try to understand what the problem is, which you'll find in some random comment inside `Program.cs`. A much better solution would be to accept the private key as a command line parameter, or possibly take it from an environment variable.

Start in the Box.V2.Samples.JWTAuth sample and inspect Program.cs, where the missing private_key.pem requirement is documented in a comment. Reproduce the clone-and-build failure, then determine how the sample should receive the key and verify that a fresh build no longer depends on an unexplained missing file.

Written by the indexing model from the issue text.