[Api Proposal] Add List<T> AsSpan to CollectionsMarshal

Its not a "safe" operation, though it is a useful one to gain access to the prebuilt array; without first copying using `.ToArray()`
```csharp
namespace System.Runtime.InteropServices
{
    public partial static class CollectionsMarshal
    {
        public Span<T> AsSpan(List<T> list);
        public Memory<T> AsMemory(List<T> list);
    }
}
```

/cc @jkotas 