Allow TypeScript dates to also be strings

[grant]

Expected Behavior

In a few types, we use the TypeScript Date field. We should allow strings with Date | string

Actual Behavior

We only use Date

Example:

google-cloudevents-nodejs/firebase/auth/v1/AuthEventData.ts

Line 82 in d54cfbe

createdAt?: Date;

[grant]

I've created a PR that fixes this.