Idea: Port validator-key-tool to Java and support HSM

[sappenin]

One day we might consider porting the validator-key-tool to java so that it would be easier to write integrations with HSM or other systems that likely have better support in Java than in C++.

While ripple/validator-keys-tool#48 is likely best implemented over in that project, it's interesting to imagine being able to interact with that workflow from Java.