Fix windows gssapi username

[rezanatha]

Fix Windows GSSAPI username domain prefix issue

Problem

On Windows systems, kerberos.authGSSClientUserName() returns domain-qualified usernames in the format DOMAIN\\user@realm.com, but SASL authentication expects just the principal part user@realm.com. This causes authentication failures when using GSSAPI on Windows.

Solution

• Add platform detection to identify Windows systems
• Strip domain prefix from username (with string as type) on Windows platform and preserving existing behavior on non-Windows platforms

All existing tests pass and tested with actual Windows domain authentication (Apache Impala Connection)

[Neustradamus]

It replaces:

• fix windows GSSAPI username domain prefix issue #45