Support using any dictionary as JsonExtensionData

IMHO, IReadOnlyDictionary should work identical to IDictionary when using with JsonExtensionData attribute.

[JsonExtensionData]
public IReadOnlyDictionary<string, JsonElement> Payload { get; set; }

Currently it fails with error:

System.InvalidOperationException: The data extension property 'Payload' does not match the required signature of IDictionary<string, JsonElement> or IDictionary<string, object>.

This is analogous to collection properties which can be defined as List<T>, IList<T> or IReadOnlyList<T> etc.