[Unique] Data annotation

[bugproof]

Currently, it's possible to ensure field uniqueness like this:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Model>()
        .HasIndex(m => m.Field)
        .IsUnique();
}

I'm not sure about other ways.

My proposition is to add [Unique] data annotation to simplify this process.

[lajones]

@midnightstarr There has been a suggestion for an Index attribute in the past. I think Unique would be part of that. So closing as a dupe of #4050.