adding getAuthTag for ciphers
adding getAuthTag for ciphers

Summary:
getAuthTag makes sense only for ciphers with GCM mode; however, that would be a little hard to encode into type system, so I am adding it for all cipher modes.
Closes https://github.com/facebook/flow/pull/1133

Reviewed By: svcscm

Differential Revision: D2749342

Pulled By: jeffmo

fb-gh-sync-id: 9f191602829826ff44deea6cbb023ea024294c31

popham/flow,AprilArcus/flow,d14b3l/flow,jgrund/flow,ylu1317/flow,gabro/flow,jdelStrother/flow,d14b3l/flow,jdelStrother/flow,ForbesLindesay/flow,mroch/flow,AprilArcus/flow,jgrund/flow,d14b3l/flow,gabelevi/flow,jamesgpearce/flow,nmote/flow,mroch/flow,nmote/flow,facebook/flow,jdelStrother/flow,facebook/flow,jdelStrother/flow,fletcherw/flow,graue/flow,jgrund/flow,shiftcars/flow,gabelevi/flow,cesarandreu/flow,ylu1317/flow,popham/flow,TiddoLangerak/flow,gabro/flow,gabelevi/flow,claudiopro/flow,nmote/flow,AgentME/flow,mroch/flow,AgentME/flow,popham/flow,AprilArcus/flow,mroch/flow,chenglou/flow,d14b3l/flow,fletcherw/flow,cesarandreu/flow,claudiopro/flow,JonathanUsername/flow,JonathanUsername/flow,facebook/flow,graue/flow,samwgoldman/flow,ForbesLindesay/flow,jamesgpearce/flow,facebook/flow,JonathanUsername/flow,JonathanUsername/flow,samwgoldman/flow,gabro/flow,ylu1317/flow,ForbesLindesay/flow,jamesgpearce/flow,chenglou/flow,gabro/flow,JonathanUsername/flow,jgrund/flow,claudiopro/flow,ForbesLindesay/flow,fletcherw/flow,jeffmo/flow,MichaelDeBoey/flow,claudiopro/flow,TiddoLangerak/flow,nmote/flow,nmote/flow,MichaelDeBoey/flow,samwgoldman/flow,gabro/flow,jeffmo/flow,jgrund/flow,cesarandreu/flow,ForbesLindesay/flow,samwgoldman/flow,popham/flow,shiftcars/flow,gabelevi/flow,TiddoLangerak/flow,MichaelDeBoey/flow,facebook/flow,MichaelDeBoey/flow,fletcherw/flow,TiddoLangerak/flow,jamesgpearce/flow,mroch/flow,jeffmo/flow,TiddoLangerak/flow,samwgoldman/flow,jdelStrother/flow,ylu1317/flow,graue/flow,popham/flow,gabelevi/flow,AgentME/flow,AprilArcus/flow,ForbesLindesay/flow,AgentME/flow,jeffmo/flow,popham/flow,AprilArcus/flow,nmote/flow,jamesgpearce/flow,fletcherw/flow,JonathanUsername/flow,d14b3l/flow,jamesgpearce/flow,ylu1317/flow,mroch/flow,jgrund/flow,cesarandreu/flow,JonathanUsername/flow,gabelevi/flow,d14b3l/flow,AgentME/flow,fletcherw/flow,AprilArcus/flow,chenglou/flow,gabelevi/flow,chenglou/flow,jdelStrother/flow,jeffmo/flow,graue/flow,chenglou/flow,graue/flow,claudiopro/flow,shiftcars/flow,chenglou/flow,shiftcars/flow,facebook/flow,facebook/flow,mroch/flow,shiftcars/flow,samwgoldman/flow,graue/flow,jamesgpearce/flow,samwgoldman/flow,MichaelDeBoey/flow,popham/flow,jeffmo/flow,shiftcars/flow,gabro/flow,nmote/flow