Summary:
To remove OpenSSL dependencies from some of our public interfaces, we need to
make `AsyncTransportCertificate` more implementation-agnostic. To start, we can
create an `OpenSSLTransportCertificate` that will have the `getX509()` method
instead of the `AsyncTransportCertificate`.

I start here by making `OpenSSLTransportCertificate` a dummy class, then I'll
update all our callsites to ensure that `getX509()` is only called on instances
of this class. After that, I can move `getX509()` exclusively to
`OpenSSLTransportCertificate`.

This means that in the future interfaces can use `AsyncTransportCertificate`
without depending on OpenSSL.

Reviewed By: yfeldblum, mingtaoy

Differential Revision: D26583479

fbshipit-source-id: 664e697a32fce61d22ee9842f998c4de1182924c