Configure Identity in a DbContext with composition

### 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.

- Trying to have an app's `DbContext` independent from `AspNetCore.Identity`
- Need the ability to inherit other custom `DbContext` that does not use `IdentityDbContext`

### Describe the solution you'd like

The capability to set up Identity entities with composition instead of inheritance, e.g. using extension method on `ModelBuilder`:

```csharp
public class AppDbContext(DbContextOptions options) : DbContext(options)
{
protected override void OnModelCreating(ModelBuilder builder)
{
base.OnModelCreating(builder);
builder.ConfigureIdentity(this); // Voilà
}
}
```

### Additional context

_No response_