Username and password for GSSAPI auth on Windows

[mstefdt]

When GSSAPI is selected as the SASL mechanism on Windows librdkafka will attempt to use Kerberos SSPI with the current user. Even if we call librdkafka while impersonated with a different user the new thread will ignore the impersonation.

To make librdkafka work with our use case I added 3 new config options, sasl.win32gssapi.username, sasl.win32gssapi.domain and sasl.win32gssapi.password that will be used for SSPI. If not provided, it will use the default credentials like before.