Switch from 400 to 403 on ImmutableAttributeError

Change-Id: Ib34903487e6a2322dd9ef3afd02793916f22207e
Simple Certificate Extension

Implement the Simple Certificate Extension for retrieving token signing
certificates from a server.

Change-Id: I653c527ca2b39628658a24b4d484683d1ac6acf2
Closes-Bug: #1259011
Merge "Switch from 400 to 403 on ImmutableAttributeError"
Merge "Simple Certificate Extension"