FR: Expose passwordUpdatedAt in auth.UserRecord

* Operating System version: Linux
* Firebase SDK version: v4.15.1
* Library version: v4.15.1
* Firebase Product: auth

### Describe the problem
I need to enforce passwords rotation and for that I need the timestamp when the password was last changed. This is available via the [REST API](https://firebase.google.com/docs/reference/rest/auth#section-get-account-info) as the `passwordUpdatedAt` property, which is my current workaround. It would be nice to have this included in the Go SDK though, so I don't have to mix raw HTTP calls with SDK.

#### Steps to reproduce:
`auth.UserRecord` struct does not have the `PasswordUpdatedAt` property. Looks like the REST parsing inside the SDK is done via the `userQueryResponse` struct in `user_mgt.go` file. That one is missing this property too.

Can you make this property available please? Thanks!