Access to arbitrary JSON meta-data

[atteneder]

The current API introduced in 6.0.0 and driven by Newtonsoft JSON gives access to meta-data on any JSON object via a TryGetValue<T>(string key) method.

The properties (i.e. their keys and value type) are not know in some use-cases and for that users need to be able to query or iterate them.

Here's some initial suggestion to add on any de-serialization class:

1. bool HasProperty(string key, out Type propertyType)
2. IEnumerable<string> GetPropertyKeys which iterates all non-known properties.
3. T GetPropertyValue<T>(string key) to retrieve a property's value.

While Newtonsoft JSON would be capable of that, the dual de-serialization class setup (GLTFast.Schema vs. GLTFast.Newtonsoft.Schema) did not allow us to add this without breaking the API or compromising bad design in the API.