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

server/auth/delegation.go

Index: server/auth/delegation.go
diff --git a/server/auth/delegation.go b/server/auth/delegation.go
index 2cc19c2f4cb3559a9eec56b6e0096199c07e2414..bd1f115ec32117fdc5b0d6426a6c728380094ef6 100644
--- a/server/auth/delegation.go
+++ b/server/auth/delegation.go
@@ -10,6 +10,8 @@ import (
 	"strings"
 	"time"
 
+	"google.golang.org/grpc"
+
 	"golang.org/x/net/context"
 
 	"github.com/luci/luci-go/common/clock"
@@ -95,7 +97,12 @@ type DelegationTokenParams struct {
 	// rpcClient is token server RPC client to use.
 	//
 	// Mocked in tests.
-	rpcClient minter.TokenMinterClient
+	rpcClient tokenMinterClient
+}
+
+// tokenMinterClient is subset of minter.TokenMinterClient we use.
+type tokenMinterClient interface {
+	MintDelegationToken(context.Context, *minter.MintDelegationTokenRequest, ...grpc.CallOption) (*minter.MintDelegationTokenResponse, error)
 }
 
 // delegationTokenCache is used to store delegation tokens in the cache.