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>

jws/jws.go

@@ -92,6 +92,9 @@ type Header struct {
 
 	// Represents the token type.
 	Typ string `json:"typ"`
+
+	// The optional hint of which key is being used.
+	KeyID string `json:"kid,omitempty"`
 }
 
 func (h *Header) encode() (string, error) {