Add NullableContextAttribute

[terrajobst]

Related to #29039

@jcouv, what's the shape for this one?

[jcouv]

Below is the shape for NullableContextAttribute and NullableMembersAttribute (copied from dotnet/roslyn#36152).

From our discussion this morning, I assume the decision for NullableAttribute applies to those as well. Namely, we'll let the compiler inject those types for now.

namespace System.Runtime.CompilerServices
{
    [System.AttributeUsage(
        AttributeTargets.Module |
        AttributeTargets.Class |
        AttributeTargets.Delegate |
        AttributeTargets.Interface |
        AttributeTargets.Method |
        AttributeTargets.Struct,
        AllowMultiple = false,
        Inherited = false)]
    public sealed class NullableContextAttribute : Attribute
    {
        public readonly byte Flag;
        public NullableContextAttribute(byte flag)
        {
            Flag = flag;
        }
    }

    public enum NullableMembers
    {
        Public = 0,   // public and protected only
        Internal = 1, // public, protected, internal
        All = 2,
    }

    [System.AttributeUsage(AttributeTargets.Module, AllowMultiple = false)]
    public sealed class NullableMembersAttribute : Attribute
    {
        public readonly NullableMembers Members;
        public NullableMembersAttribute(NullableMembers members)
        {
            Members = members;
        }
    }
}

FYI @cston

[terrajobst]

Closing, because we closed #29039 too.