Add support for PKCS12 & PKCS7 certificate formats to Schannel TLS

[wfurt]

#1351 added option to load certificate from ASN.1 encoded buffer and #1450 allows to export peer certificate(s) in ASN.1 form as well.
While there is no pressing need for this on Windows with Schannel, it would be nice to get parity for anybody who does not want to or cannot use platform pointers.

[nibanks]

@wfurt what is the priority here? Is this a requirement for .NET 7?

[wfurt]

low. We don't plan to support portable certificates (to OpenSsl) on Windows AFAIK.
cc: @ManickaP

[nibanks]

Long term, we want to standardize on OpenSSL instead of adding features like this to Schannel. @wfurt any issue with closing this in favor of that long term direction?

[wfurt]

From my perspective this is only for completeness. for schannel we use native pointers. So I think this is ok to close regardless of the OpenSSL.