[FR] Auth: Update user details and custom claims in the same request

Currently, the custome user claims can be updated with [`setCustomUserClaims()`](https://firebase.google.com/docs/reference/admin/node/firebase-admin.auth.baseauth.md#baseauthsetcustomuserclaims), and the user details can be updated with [`updateUser()`](https://firebase.google.com/docs/reference/admin/node/firebase-admin.auth.baseauth.md#baseauthupdateuser).

Both use same [`/accounts:update`](https://cloud.google.com/identity-platform/docs/reference/rest/v1/accounts/update) REST API under the hood.

Is there a particular reason why the `updateUser()` method does not allow updating the custom claims even though it is possible in the underlying REST API? A follow up point is: if the `updateUser()` method has this functionalitly, then the `setCustomUserClaims()` would become obsolete (just a convenience wrapper).

I'd like to be able to include a `customClaims` field in the [`UpdateRequest`](https://firebase.google.com/docs/reference/admin/node/firebase-admin.auth.updaterequest.md#updaterequest_interface) interface, so that, if needed, we can update the user's details and claims in a single request. Currently we need two separate network requests to do this.

Let me know, and I can help with a PR if you want.