Support C# 8.0 nullable reference types in model building

[MarcoLoetscher]

C# 8 is on the way and better to early than too late should be planned the implementation of the new features (Introducing Nullable Reference Types in C#).

public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; } //In C# 8 this should be required by default
    public string? Url2 { get; set; } //In C# 8 this should be optional by default

    //Needs EF support for constructors with parameters
    public Blog(string url) 
    {
        Url = url;
    }
}

A new logic is needed for mapping of e. g. string: Required and Optional Properties

Support for constructors with Parameters is required: Flexible mapping to CLR types and members (Custom O/C Mapping #240)

This list is probably not complete.

[ajcvickers]

@MarcoLoetscher This is absolutely something we want to leverage, but since we are generating additional semantics and behavior based on nullability we will need to consider carefully how to do this without making unintended breaking changes to model/schema generation.