Supports TLS alerts for cert validation for non-Windows platforms

[CIPop]

Tracking TLS Alert TODO items for non-Windows platforms (#11489,

1. close_notify implementation
2. TLS handshake implementation: please note that in order to extract the exact error during handshake, Win32Exception.NativeErrorCode is used (e.g. SEC_E_ILLEGAL_MESSAGE)

[Priya91]

The windows functionality will be available in 2.0.0 but missing in Unix. Adding it to 2.0.0, to consider this Unix work if manpower available.

Next Steps: Search issue number in System.Net.Security and implement the pal layer for Unix.

[Priya91]

The close_notify alerts are implemented. The other part of implementing cert failure alerts is tricky, as openssl doesn't expose any APIs to do that. The given structure of cert validation happens after the handshake. To support this, we will have to change this order, and make the cert validation be a part of handshake, by registering a certificate validation callback with openssl. The registered callback has to return failure upon which the openssl will send the TLS alerts.

This work involves restructuring the pal error in System.Net.Security, and cert failure alerts is a nice to have feature. Moving to future.

[karelz]

There is specific issue: #914

We should review the disabled tests, see what we can reasonably fix and then close this in 5.0.

[wfurt]

triage: may be fixed with new OpenSSL. We should verify and possibly close for 8.0