(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 suppose just leave it, but it would certainly be better to have the static initializer in this class rather than in a special sub-class of `HashMap` that may or may not be redefined each time this field is accessed.