Adds GSSManager factory class, to be used for SPNEGO HTTP authentication

Adds GSSManager factory class, to be used for SPNEGO HTTP authentication

Adds a factory class for GSSManager. GSSManager is already a factory class but
it doesn't work well with SPNEGO HTTP authentication unless the
javax.security.auth.useSubjectCredsOnly system property is set to false.

On Sun JDK, Sun GSSManagerImpl can be configured directly for SPNEGO auth
(without setting any system property). For a better experience, the class
availability is detected by reflection and used instead.

Detection is only done once, and fallbacks to org.ietf.jgss.GSSManager.

Bug: 428836
Change-Id: Idb9123d1f4013966919db43043ec959d4b133ae2
Signed-off-by: Laurent Goujon <lgoujon@twitter.com>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>