(PE-4372) Added utilities for alt dns names

* Able to add extensions to a CSR
* Added utilities to create an Alt. DNS names extensions

This PR is for review only so people can start taking a look at it while I write some tests.
I might prefer Java-style conventions for the Java-side of this library, so the name would be something like `createX500Name`, `generateX500Name`, `newX500Name`, `x500NameFromFoo`, `fooToX500Name`, or something.

Not a blocker, just looks kinda weird?