This PR adds unconstrained Kerberos delegation to the API.
The client obtains a forwarded TGT and sends it, base64-encoded, in an HTTP header named 'X-KRB5-CRED'. The server reads this credential, creates a new credentials cache for the user, and stores the credential into the new cache. The server can now authenticate to other services (e.g. LDAP) over GSSAPI using the forwarded client's credentials.