Type Alias GssTokenProvider 

pub type GssTokenProvider = fn(EnterpriseAuthMechanism, Option<&[u8]>) -> Result<Vec<u8>, String>;

Callback used to generate GSS/SSPI response tokens.

The callback receives:

• negotiated enterprise auth mechanism
• optional server challenge bytes (None for initial token)

It must return the client response token bytes to send in GSSResponse.