AddUserStore should allow injection via IUserStore<T> as well as actual type

For identity services such as UserManager or RoleManager, when using a custom type, the service is available under both the UserManager AND the custom type itself for dependency injection. Similarly RoleManager and a few others also forward the services for both the required type and the custom type.
However other identity services dont, specifically the IUserStore.
In my application, I have some additional methods added to IUserStore so I need it under a more specific type, and it requires a
`services.AddScoped(typeof(CustomStore), serviceFinder => serviceFinder.GetRequiredService(typeof(IUserStore)));`
It just seems inconsistent that I have to forward the IUserStore service manually, but the UserManager and SignInManager and RoleManager, ect don't need to be.

Interestingly enough, the AddUserStore() method also isn't doing any type validation whereas all the other methods do.

### Describe the solution you'd like
I would like the implementation of AddUserStore()
https://github.com/aspnet/AspNetCore/blob/master/src/Identity/Extensions.Core/src/IdentityBuilder.cs#L108
to look more like that of AddUserManager()
https://github.com/aspnet/AspNetCore/blob/master/src/Identity/Extensions.Core/src/IdentityBuilder.cs#L145