Support for unique phone numbers, find user by phone number in Identity

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

I am trying to create users with unique phone numbers.

I know it's already mentioned here in this issue [25368](https://github.com/dotnet/aspnetcore/issues/25368) and many others, but I think it makes sense to have it now.

### Describe the solution you'd like

As suggested in the issue I referenced above, I think there could be a provided flag when creating a user, to enforce this (or not):

`await userManager.CreateAsync(user, request.Password, requireUniquePhoneNumber = true) // default could be false`

or whichever way you guys deem clean.

Then a `FindByPhonenumber` method in the `userManager`.
### Additional context

In some countries, user identity is tied to their phone numbers, so some apps require users to register with them.