[feature request] Expose SslConnector

[mrdgo]

Dear maintainers,

I do research on web security. In particular, I want to create an extension within TLS. Sadly in:

pub struct TlsConnector {
    connector: SslConnector,
    ...
}

the connector is not accessible, so I can't use your library. I currently forked your repo, just to call connector.add_custom_ext(...) in TlsConnector::new. I don't depend on this at all, I just think that this might be a nice feature to support.

I understand, if this is out of scope, because the implementation will likely be different for windows and MacOS. But maybe someone can find a way to unify this interface across platforms.

I think that this would be a valuable addition, but I am obviously biased.

Kind regards,
Maxim