Add hostnames: csr -> string list for obtaining list of domains of a csr. about ocaml-x509

[hannesm]

sadly it is not trivial, since I fail to find how it is encoded (https://tools.ietf.org/html/rfc2986#page-7 specifies attributes, maybe they hide it in there)... will need to dig into how openssl actually does it at some point... could you in the meantime use the subject (from: https://mirleft.github.io/ocaml-x509/X509.CA.html#TYPErequest_info), which is a distinguished name which may contain a single domain name as its common name ``CN` part...

[hannesm]

since I needed this functionality as well, and it is a bit tedious to write (check for SAN extension in CSR, check subject) I'd be keen to adapt it (see code at https://github.com/mmaker/ocaml-letsencrypt/blob/v0.1.0/src/acme_common.ml#L9); also X509.Certificate.hostnames is present, and a complementary X509.Signing_request.hostnames would be welcome (also resulting in a Domain_name.Set.t)