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

tokenserver/appengine/impl/serviceaccounts/rpc_mint_oauth_token_via_grant.go

+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package serviceaccounts
+
+import (
+        "golang.org/x/net/context"
+        "google.golang.org/grpc"
+        "google.golang.org/grpc/codes"
+
+        "github.com/luci/luci-go/tokenserver/api/minter/v1"
+)
+
+// MintOAuthTokenViaGrantRPC implements TokenMinter.MintOAuthTokenViaGrant
+// method.
+type MintOAuthTokenViaGrantRPC struct {
+}
+
+// MintOAuthTokenViaGrant produces new OAuth token given a grant.
+func (r *MintOAuthTokenViaGrantRPC) MintOAuthTokenViaGrant(c context.Context, req *minter.MintOAuthTokenViaGrantRequest) (*minter.MintOAuthTokenViaGrantResponse, error) {
+        return nil, grpc.Errorf(codes.Unavailable, "not implemented")
+}