Add GSS/SSPI compatible interface implementation for easier interop between systems

**Is your feature request related to a problem? Please describe.**
Most applications do not use Kerberos directly. They often rely on the platform below them exposing an interface for requesting tickets. This is GSS generically, and SSPI on Windows. It's defined in [RFC 2743](https://tools.ietf.org/html/rfc2743).

**Describe the solution you'd like**

An interface and implementation that exposes all the required functions of GSS:

```C#
interface IGssContext
{
GSS_Acquire_cred(...);
GSS_Release_cred(...);
GSS_Inquire_cred(...);

GSS_Add_cred(...);
GSS_Inquire_cred_by_mech(...);


GSS_Init_sec_context(...);
GSS_Accept_sec_context(...);
GSS_Delete_sec_context(...);
GSS_Process_context_token(...);
GSS_Context_time(...);
GSS_Inquire_context(...);
GSS_Wrap_size_limit(...);
GSS_Export_sec_context(...);
GSS_Import_sec_context(...);

GSS_GetMIC(...);
GSS_VerifyMIC(...);
GSS_Wrap(...);
GSS_Unwrap(...);

GSS_Display_status(...);
GSS_Indicate_mechs(...);
GSS_Compare_name(...);
GSS_Display_name(...);
GSS_Import_name(...);
GSS_Release_name(...);
GSS_Release_buffer(...);
GSS_Release_OID_set(...);
GSS_Create_empty_OID_set(...);
GSS_Add_OID_set_member(...);
GSS_Test_OID_set_member(...);
GSS_Inquire_names_for_mech(...);
GSS_Inquire_mechs_for_name(...);
GSS_Canonicalize_name(...);
GSS_Export_name(...);
GSS_Duplicate_name(...);
}
```