type Options struct {
	// Tp is the source credential used to generate a token on the
	// impersonated service account. Required.
	Tp auth.TokenProvider

	// URL is the endpoint to call to generate a token
	// on behalf of the service account. Required.
	URL string
	// Scopes that the impersonated credential should have. Required.
	Scopes []string
	// Delegates are the service account email addresses in a delegation chain.
	// Each service account must be granted roles/iam.serviceAccountTokenCreator
	// on the next service account in the chain. Optional.
	Delegates []string
	// TokenLifetimeSeconds is the number of seconds the impersonation token will
	// be valid for. Defaults to 1 hour if unset. Optional.
	TokenLifetimeSeconds int
	// Client configures the underlying client used to make network requests
	// when fetching tokens. Required.
	Client *http.Client
}