google: support key ID in JWTAccessTokenSourceFromJSON

Change-Id: I20ffede5bf81aa4990afb2820561d5633cdb43a8
Reviewed-on: https://go-review.googlesource.com/24440
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

jwt/jwt.go

@@ -46,6 +46,10 @@ type Config struct {
 	//
 	PrivateKey []byte
 
+	// PrivateKeyID contains an optional hint indicating which key is being
+	// used.
+	PrivateKeyID string
+
 	// Subject is the optional user to impersonate.
 	Subject string