Add constrained delegation support

I am able to obtain a service ticket for SQL Server using an impersonated credential with Java 8 but when I try to use this with the driver I am getting the next error:

KrbException: The ticket isn’t for us (35)

It seems that the problem is in the KerbAuthentication class inside the method intAuthInit(). In this class a context is created for the authentication and this context requests that the initiator’s credentials be delegated to the acceptor during context establishment. This behavior is not possible within a constrained delegation scenario.

The service ticket obtained using the impersonated credential is not a forwardable ticket and is not usable for the driver. Changing the next set to false makes the authentication works.

peerContext.requestCredDeleg(true);

I understand this is not the right change since the SQL Server can need to use a delegated credential in order to connect to other SQL Server instances using open delegation, but a change is necessary to set this property to false in a constrained delegation scenery.

[Suraiya-Hameed]

Support for constrained delegation is added in #178. Can you please review and test it?

You can add GSSCredential to the driver connection property and pass impersonated credential as value.

[jacobovazquez]

I tried both, 4.1 and 4.2, and it is working. Thanks!