Create types for untyped nodes

[MartinM85]

Closes #175

[MartinM85]

I've made changes in kiota-serialization-json-dotnet, but need to reference kiota-abstractions-dotnet 1.7.6 before creating a pull request.

Suppose that at the end, the SDK generator will generate something like this

    public class WorkbookFilterCriteria : IAdditionalDataHolder, IBackedModel, IParsable {
...
        public UntypedNode? Values {
            get { return BackingStore?.Get<UntypedNode?>("values"); }
            set { BackingStore?.Set("values", value); }
        }
...
        public virtual IDictionary<string, Action<IParseNode>> GetFieldDeserializers() {
            return new Dictionary<string, Action<IParseNode>> {
...
                {"values", n => { Values = n.GetUntypedValue(); } },
            };
        }
        public virtual void Serialize(ISerializationWriter writer) {
            _ = writer ?? throw new ArgumentNullException(nameof(writer));
...
            writer.WriteUntypedValue("values", Values);
            writer.WriteAdditionalData(AdditionalData);
        }
    }

instead of the current:

  public class WorkbookFilterCriteria : IAdditionalDataHolder, IBackedModel, IParsable {
...

        public Json? Values {
            get { return BackingStore?.Get<Json?>("values"); }
            set { BackingStore?.Set("values", value); }
        }
...
        public virtual IDictionary<string, Action<IParseNode>> GetFieldDeserializers() {
            return new Dictionary<string, Action<IParseNode>> {
...
                {"values", n => { Values = n.GetObjectValue<Json>(Json.CreateFromDiscriminatorValue); } },
            };
        }
        public virtual void Serialize(ISerializationWriter writer) {
            _ = writer ?? throw new ArgumentNullException(nameof(writer));
...
            writer.WriteObjectValue<Json>("values", Values);
            writer.WriteAdditionalData(AdditionalData);
        }
    }

[baywet]

Thanks for the contribution, a couple of remarks. I think you can implement the POC with a local reference for the json serialization and see how that turns out.

[andrueastman]

Just a comment on this side.
Otherwise, we need a version bump to 1.8.0 as well as a changelog entry.

[github-actions]

Conflicts have been resolved. A maintainer will take a look shortly.

[github-actions]

This pull request has conflicting changes, the author must resolve the conflicts before this pull request can be merged.

[baywet]

Just as an update to anyone watching this: we're finalizing the work on the other main languages before we merge everything in. Thank you for your patience :)