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

tokenserver/client/tokenclient.go

Index: tokenserver/client/tokenclient.go
diff --git a/tokenserver/client/tokenclient.go b/tokenserver/client/tokenclient.go
index c64632db804f6d8d93a653fbafbd3a7c08819be2..1fb90b9d9eec00146a1e8b460e29ead6deb9ff7c 100644
--- a/tokenserver/client/tokenclient.go
+++ b/tokenserver/client/tokenclient.go
@@ -37,12 +37,18 @@ type Client struct {
 	//
 	// Use minter.NewTokenMinterClient (or NewTokenMinterPRPCClient) to
 	// create it. Note that transport-level authentication is not needed.
-	Client minter.TokenMinterClient
+	Client TokenMinterClient
 
 	// Signer knows how to sign requests using some private key.
 	Signer Signer
 }
 
+// TokenMinterClient is subset of minter.TokenMinterClient this package uses.
+type TokenMinterClient interface {
+	// MintMachineToken generates a new token for an authenticated machine.
+	MintMachineToken(context.Context, *minter.MintMachineTokenRequest, ...grpc.CallOption) (*minter.MintMachineTokenResponse, error)
+}
+
 // Signer knows how to sign requests using some private key.
 type Signer interface {
 	// Algo returns an algorithm that the signer implements.