Implement Workload API

[drrt]

From @kunzimariano on August 7, 2017 20:51

GRPC (Service side).

Copied from original issue: spiffe/node-agent#31

[drrt]

From @y2bishop2y on August 8, 2017 4:40

Removing from Repo Project. Should just be in the Org project

[evan2645]

We don't yet know exactly what this will look like. Pending some decisions on FederatedRoot vs FederatedBundle. Closing until we figure that out.