(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.
Yikes. This will allow users to manually specify their own SubjectAltName field so this is something I'm uncomfortable having exist in the codebase at all. I think that you should filter for the specific SubjectAltName OID now and only allow that; that's what Puppet did before csr_attributes came along. It'll be simple enough to loosen the restrictions later.