Parsing Subject Alternative Names (SANs) from X509Certificate

Hello and thanks for the great crate,

I was wondering if fetching Subject Alternative Names (SANs) from X509 extensions (e.g. from the `X509Certificate::iter_extensions()` iterator) is a feature that's somewhat supported. I'm guessing that my best bet would be to match specific object IDs ([Oid](https://docs.rs/bcder/0.7.4/bcder/oid/struct.Oid.html)) from the [Extension struct](https://docs.rs/x509-certificate/latest/x509_certificate/rfc5280/struct.Extension.html), and then parsing the output bytes as UTF8 characters with something like `String::from_utf()`.

Is there a better approach?

Start with X509Certificate::iter_extensions() and the RFC5280 Extension and bcder Oid types named in the issue. Confirm whether SANs are already exposed; if not, define the parsing and API scope with maintainer guidance. Done should mean a clear supported approach for extracting SAN values, backed by an acceptance case.

Written by the indexing model from the issue text.