proto: Change RestartPolicy variants to use CamelCase

This is a backwards-incompatible change, which is fine because we
haven't released a version of go-tpm-tools containing `RestartPolicy`.
This change also means that the variants of RestartPolicy no longer
conform to the Protobuf style guide. An explanation for why we do this
is given in the comments in the `attest.proto` file.

Signed-off-by: Joe Richey <joerichey@google.com>

proto/attest.proto

@@ -115,13 +115,15 @@ message SecureBootState {
 }
 
 // The container's restart policy.
-// See
+// See the following Kubernetes documentation for more details:
 // https://kubernetes.io/docs/concepts/workloads/pods/pod-lifecycle/#restart-policy
-// for more details.
+// Note that these enum variants do not conform to the standard Protocol Buffers
+// style guide so that RestartPolicy_name, RestartPolicy_value, and
+// RestartPolicy.String() match the values used by Kubernetes and others.
 enum RestartPolicy {
-  ALWAYS = 0;
-  ON_FAILURE = 1;
-  NEVER = 2;
+  Always = 0;
+  OnFailure = 1;
+  Never = 2;
 }
 
 message ContainerState {