Default Values for IdentityUser

I've realized that the configuration for the default entity `IdentityUser`
https://github.com/aspnet/Identity/blob/master/src/EF/IdentityUserContext.cs#L134 doesn't have any configuration for the `PhoneNumber` and other values.

I faced a few performance lacking using varchar(max) sizing.

Reference : https://sqlperformance.com/2017/06/sql-plan/performance-myths-oversizing-strings

I think it'd be a good idea use some standard as default to avoid users that don't care or don't know about it have to deal with those kind of throughput issues and every once can configure like they prefer (as it is possible now) but IMO is a quite dangerous letting this as default


Acording this with `For full international support, you'd need a VARCHAR of 15 digits.`
https://en.wikipedia.org/wiki/E.164

Thanks.