Add access methods to Certificate

[vaffeine]

Hi,

I need to access server's public key to use it in the NTLM implementation that I am working on.

To do so, I use Certificate::to_der() method and then parse the public key using X509 from OpenSSL. But this, obviously, is not really convenient, because the key is already parsed by native_tls and I need to parse it again with platform dependent code.

I think it would be great to have methods to access certificate fields in a platform-independent way. X509 from OpenSSL provides a lot of methods for it. It is also possible to retrieve some of them with SecCertificateExt on macOS and the PR, that allows to retrieve public certificate, is almost merged into schannel-rs .

I could work on it and create a PR if you don't mind.

[sfackler]

Sure - PRs exposing more bits of the system are welcome!

[vaffeine]

Oh, I actually looked into exising PRs and found, that SergejJurecko has already done simmilar thing in #117. I'll wait until his PR is merged and then try to add more methods.