Update CloudEvents SDK to support most recent spec change to extensions#2

Merged

kars7e merged 2 commits intocloudevents:masterfrom tenczar:topic/ntenczar/map_based

Oct 17, 2018

+1,216-236

Commits on Sep 20, 2018

Updates CloudEvents SDK to support top level extension with various serializers/deserializers
tenczar
committed

Commits on Sep 28, 2018

Finalizing changes for cloudevents contribution
tenczar
committed