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

tokenserver/api/machine_token.pb.go

 // Code generated by protoc-gen-go.
 // source: github.com/luci/luci-go/tokenserver/api/machine_token.proto
 // DO NOT EDIT!
 
 /*
 Package tokenserver is a generated protocol buffer package.
 
 It is generated from these files:
         github.com/luci/luci-go/tokenserver/api/machine_token.proto
+        github.com/luci/luci-go/tokenserver/api/oauth_token_grant.proto
         github.com/luci/luci-go/tokenserver/api/token_file.proto
 
 It has these top-level messages:
         MachineTokenBody
         MachineTokenEnvelope
+        OAuthTokenGrantBody
+        OAuthTokenGrantEnvelope
         TokenFile
 */
 package tokenserver
 
 import proto "github.com/golang/protobuf/proto"
 import fmt "fmt"
 import math "math"
 
 // Reference imports to suppress errors if they are not otherwise used.
 var _ = proto.Marshal
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