Support for Mutual TLS

[ripavymsft]

Context:

• Client and server each initialize SchannelCred with schannel_cred::SchannelCred::builder().cert(certificate). Client and server each have their own cert.

Observed behavior:

• the TlsStream on the client-side shows: client_tls_stream.peer_certificate() returns the server certificate, as expected
• the TlsStream on the server-side has: server_tls_stream.peer_certificate() returns an Err value: Os { code: -2146893042, kind: Uncategorized, message: "No credentials are available in the security package"

Expected behavior:

• The server_tls_stream.peer_certificate() should show the client certificate and the TLS connection is authenticated on both sides.