[API Proposal]: Add a generic ReadOnlyOrderedDictionary class

### Background and motivation

We now have a generic `OrderedDictionary` and a `ReadOnlyDictionary` but no combination of the two. We have need of such a data structure where the key order is important and we also want it to be immutable.

### API Proposal

```csharp
namespace System.Collections.Generic;

public class ReadOnlyOrderedDictionary :
IDictionary, IReadOnlyDictionary, IDictionary,
IList>, IReadOnlyList>, IList
where TKey : not null
{
// Same methods/properties as ReadOnlyDictionary, but enumerations are ordered
}
```

### API Usage

```csharp
var dict = new ReadOnlyOrderedDictionary(
[
new KeyValuePair("foo","bar"),
new KeyValuePair("foo1","bar2"),
]);

foreach ((var key, var value) in dict)
{
// ...
}
```

### Alternative Designs

_No response_

### Risks

_No response_