Interop Guidance should contain enum types

Current interop guidance does not include any information about enums.

But enums with same underlying type seems to be blittable. For example the following works in C# , and it should not be a problem when corresponding structure in C or C++ has also the same types and layout.:

```csharp

enum Foo : int { A, B, C }
enum Bar : short { A, B, C }

[StructLayout(LayoutKind.Sequential)]
struct Baz
{
public Foo Foo;
public Bar Bar;
}

class Program
{
static void Main(string[] args)
{
var x = new Baz { Foo = Foo.B, Bar = Bar.C };
Blit(x, out var y);
Console.WriteLine($"{y.Foo}/{y.Bar}");
}

private unsafe static void Blit(in TValue x, out TValue y)
where TValue : unmanaged
{
fixed (byte* inval = (byte*)&x)
fixed (byte* outval = (byte*)&y)
{
int size = Unsafe.SizeOf();
for (int i = 0; i < size; ++i)
{
outval[i] = inval[i];
}
}
}
}

```

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* ID: 6ea501d3-d963-c5dc-342d-ee82c92d1850
* Version Independent ID: e58f3d16-8083-0efe-0057-e21dbebb262f
* Content: [Native interoperability best practices - .NET](https://docs.microsoft.com/en-us/dotnet/standard/native-interop/best-practices)
* Content Source: [docs/standard/native-interop/best-practices.md](https://github.com/dotnet/docs/blob/main/docs/standard/native-interop/best-practices.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @jkoritzinsky
* Microsoft Alias: **jekoritz**