token-server: Add protos for new API for generating service account tokens.

tokenserver/client/tokenclient_test.go

Index: tokenserver/client/tokenclient_test.go
diff --git a/tokenserver/client/tokenclient_test.go b/tokenserver/client/tokenclient_test.go
index 5b270ba7f08398b68a4b5f780c694acc4a88d0b1..48c6d3d4bf9a8ea83a82d0b33f26b720dd7434d3 100644
--- a/tokenserver/client/tokenclient_test.go
+++ b/tokenserver/client/tokenclient_test.go
@@ -103,10 +103,6 @@ func (f *fakeRPCClient) MintMachineToken(ctx context.Context, in *minter.MintMac
 	return &f.Out, nil
 }
 
-func (f *fakeRPCClient) MintDelegationToken(context.Context, *minter.MintDelegationTokenRequest, ...grpc.CallOption) (*minter.MintDelegationTokenResponse, error) {
-	panic("not implemented")
-}
-
 // fakeSigner implements Signer.
 type fakeSigner struct{}