Add a service to configure conventions

It will be similar to `IModelCustomizer`:

```C#
public interface IConventionCustomizer
{
///
/// Sets model defaults and configures conventions before they run for a given context.
///
///
/// If any instance data from is
/// used when building the model, then the implementation of
/// also needs to be updated to ensure the model is cached correctly.
///
///
/// The builder being used to set defaults and configure conventions that will be used to build the model for this context.
///
///
/// The context instance that the model is being created for.
///
void Customize(ModelConfigurationBuilder configurationBuilder, DbContext context);
}
```

The default implementation will call `context.ConfigureConventions(configurationBuilder)`;