Automatic merge from submit-queue (batch tested with PRs 45709, 41939)

Add an AEAD encrypting transformer for storing secrets encrypted at rest

Tweak the ValueTransformer interface slightly to support additional
context information (to allow authenticated data to be generated by the
store and passed to the transformer). Add a prefix transformer that
looks for known matching prefixes and uses them. Add an AES GCM
transformer that performs AEAD on the values coming in and out of the
store.

Implementation of https://docs.google.com/document/d/1lFhPLlvkCo3XFC2xFDPSn0jAGpqKcCCZaNsBAv8zFdE/edit# and https://github.com/kubernetes/kubernetes/issues/12742