(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.
what would those strings look like?  Would it be worth making an assertion that the value is the expected string?  As-is the test just looks like it doesn't consider that to be important :)