Warn on Migrate() when there are pending model changes

[AndriySvyryd]

It should throw by default.

Related to #22105

[topstop3416]

asp.net core configuration provider that reads configuration key-value pairs from a database using Entity Framework Core

dotnet ef migrations add throw PendingModelChangesWarning

https://learn.microsoft.com/en-us/aspnet/core/fundamentals/configuration/?view=aspnetcore-9.0#custom-configuration-provider

public class EFConfigurationProvider : ConfigurationProvider
{
    public EFConfigurationProvider(Action<DbContextOptionsBuilder> optionsAction)
    {
        OptionsAction = optionsAction;
    }

    Action<DbContextOptionsBuilder> OptionsAction { get; }

    public override void Load()
    {
        var builder = new DbContextOptionsBuilder<EFConfigurationContext>();

        OptionsAction(builder);

        using (var dbContext = new EFConfigurationContext(builder.Options))
        {            
            dbContext.Database.Migrate();    // throw PendingModelChangesWarning when dotnet ef migrations add

            Data = dbContext.Values.ToDictionary(c => c.Id, c => c.Value);
        }
    }
}