(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.
That's not a bad idea. At this point, I think we (read "I") aren't handling extensions in the best manner here and a reworking would not be a bad idea at all.