Add support for serializing types that implement IDictionary<string, object>

[layomia]

This fixes https://github.com/dotnet/corefx/issues/38372. cc @davidfowl @MarcoRossignoli

public static void ImplementsIDictionaryOfObject()
{
    var input = new StringToObjectIDictionaryWrapper(new Dictionary<string, object>
    {
        { "Name", "David" },
        { "Age", 32 }
    });

    string json = JsonSerializer.ToString(input, typeof(IDictionary<string, object>));
    Assert.Equal(@"{""Name"":""David"",""Age"":32}", json); // Assertion passes.
}

public class StringToObjectIDictionaryWrapper : IDictionary<string, object>
{
    private readonly IDictionary<string, object> _dict;

    public StringToObjectIDictionaryWrapper(IDictionary<string, object> dict)
    {
        _dict = dict;
    }

    public object this[string key] { get => _dict[key]; set => _dict[key] = value; }

    public ICollection<string> Keys => _dict.Keys;

    public ICollection<object> Values => _dict.Values;

    public int Count => _dict.Count;

    public bool IsReadOnly => false;

    public void Add(string key, object value)
    {
        _dict.Add(key, value);
    }

    public void Add(KeyValuePair<string, object> item)
    {
        _dict.Add(item.Key, item.Value);
    }

    public void Clear()
    {
        _dict.Clear();
    }

    public bool Contains(KeyValuePair<string, object> item)
    {
        return _dict.Contains(item);
    }

    public bool ContainsKey(string key)
    {
        return _dict.ContainsKey(key);
    }

    public void CopyTo(KeyValuePair<string, object>[] array, int arrayIndex)
    {
        _dict.CopyTo(array, arrayIndex);
    }

    public IEnumerator<KeyValuePair<string, object>> GetEnumerator()
    {
        return _dict.GetEnumerator();
    }

    public bool Remove(string key)
    {
        return _dict.Remove(key);
    }

    public bool Remove(KeyValuePair<string, object> item)
    {
        return _dict.Remove(item);
    }

    public bool TryGetValue(string key, out object value)
    {
        return _dict.TryGetValue(key, out value);
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

To-do: Extend support such that we don't have to indicate the implemented type (https://github.com/dotnet/corefx/issues/38521) i.e

public static void ImplementsIDictionaryOfObject()
{
    var input = new StringToObjectIDictionaryWrapper(new Dictionary<string, object>
    {
        { "Name", "David" },
        { "Age", 32 }
    });

    string json = JsonSerializer.ToString(input); // Type not indicated.
    Assert.Equal(@"{""Name"":""David"",""Age"":32}", json); // Assertion passes.
}

This PR also adds support for (de)serializing IDictionary<string, object>.

[JeremyKuhne]

Just a nit, otherwise looks good.

[steveharter]

One comment around the original code - please look at history and see why that code was added to make sure we don't break something else. Thanks

[layomia]

One comment around the original code - please look at history and see why that code was added to make sure we don't break something else. Thanks

See https://github.com/dotnet/corefx/pull/37300/files#r280149022.
It's preventing an attempt to (de)serialize object types. Now that JsonElement is in, we can lift this guard as we need to get a concrete type for IDictionary<string, object> (-> Dictionary<string, object>)

[ahsonkhan]

Why remove the IDictionary test case rather than just adding the Dictionary<string, object> one?