jwt.go
func PeekPayloadField(buf []byte, field string) (string, error) {
	return peekField(buf, field, tokenPositionPayload)
}
func PeekPayloadField(buf []byte, field string) (string, error) {
	return peekField(buf, field, tokenPositionPayload)
}
// PeekPayloadField extracts the specified field from the serialized JWT buf's
// payload (ie, the token claims). An error will be returned if the field is
// not present in the decoded payload.