Enable TLS channel binding token for NTLM authentication on Linux (dotnet/corefx...

Enable TLS channel binding token for NTLM authentication on Linux (dotnet/corefx#34998)

This PR adds support for passing in the computed TLS channel binding
token (cbt) into the gssapi during NTLM authentication. This included
removing the code that threw the PlatformNotSupported exception.

No new tests were added to this PR since it requires machine changes
to test. However, I manually tested various combinations of NTLM
authentication including when the server requires (or NOT) the cbt
to be used. Currently only Windows Servers uses this functionality
as part of the "Extended Protection for Authentication" feature.

CurlHandler does not support using the cbt during NTLM authentication.
This is a problem with libcurl itself.

As part of this PR I removed a redundant assert and refactored the
SecChannelBindings structure into src/Common.

Fixes dotnet/corefx#34879

Commit migrated from https://github.com/dotnet/corefx/commit/6575de65608c95717b2a032b689adae32378778f