Enable nullable backing field pattern for store-generated defaults

[ajcvickers]

See discussion here: #15070 (comment)

Enable this:

public class Blog
{
    private bool? _isValid;

    public bool IsValid
    {
        get => _isValid ?? false;
        set => _isValid = value;
    }
}

and

    .HasDefaultValue(true);

(There should be no need to set the property access mode since we read/write fields by default in 3.0.)

This pattern means:

• The column has a default constraint in the database
• If the property is never set, then no value will be included in the insert, and hence the store default will be used.
• If the property is set to anything (true or false) in this example, then that value will be explicitly included in the insert.

[ajcvickers]

Consider #12215 when working on this.

[ajcvickers]

See also scenario in #20148