(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.
Not particularly thrilled with 2-letter acronym function names. I would consider using the full name instead of the acronym, or if it makes sense to follow the `thisType->thatType` convention.