tokenserver: Implement MintOAuthTokenGrant RPC.

tokenserver/appengine/impl/services/minter/tokenminter/service.go

 // Copyright 2016 The LUCI Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //      http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 36 matching lines @@
                 MintMachineTokenRPC: machinetoken.MintMachineTokenRPC{
                         Signer:           gaesigner.Signer{},
                         CheckCertificate: certchecker.CheckCertificate,
                         LogToken:         machinetoken.LogToken,
                 },
                 MintDelegationTokenRPC: delegation.MintDelegationTokenRPC{
                         Signer:   gaesigner.Signer{},
                         Rules:    delegation.GlobalRulesCache.Rules,
                         LogToken: delegation.LogToken,
                 },
+                MintOAuthTokenGrantRPC: serviceaccounts.MintOAuthTokenGrantRPC{
+                        Signer: gaesigner.Signer{},
+                        Rules:  serviceaccounts.GlobalRulesCache.Rules,
+                },
         }
 }