fix: add EmbeddedObject and EmbeddedObjectConverter to resolve IEmbeddedObject deserialization failure

[OMpawar-21]

Problem

Calling .includeEmbeddedItems().Fetch<T>() on a model that implements
IEntryEmbedable throws a JsonSerializationException at runtime:

Could not create an instance of type Contentstack.Utils.Interfaces.IEmbeddedObject.
Type is an interface or abstract class and cannot be instantiated.
Path 'entry._embedded_items.rte_json[0]._content_type_uid'

IEntryEmbedable declares embeddedItems as
Dictionary<string, List<IEmbeddedObject>>. When the Delivery API returns
_embedded_items in the response, Newtonsoft.Json tries to instantiate
IEmbeddedObject directly. Because it is an interface, instantiation fails.

Changes

Contentstack.Utils/Models/EmbeddedObject.cs (new)

Concrete class implementing both IEmbeddedEntry and IEmbeddedAsset.
Newtonsoft.Json can instantiate this in place of IEmbeddedObject.
[JsonExtensionData] on the Fields property preserves any custom fields
on the embedded entry or asset (e.g. bio, email, avatar_url) that are
not explicitly declared.

Contentstack.Utils/Converters/EmbeddedObjectConverter.cs (new)

JsonConverter that intercepts IEmbeddedObject at deserialization time
and redirects to EmbeddedObject. CanConvert uses exact type equality
(==) rather than IsAssignableFrom so consumer-defined subclasses of
IEmbeddedObject are not intercepted.

Consumer Impact

No changes required in consumer code. Models implementing IEntryEmbedable
continue to work as-is after the delivery SDK registers EmbeddedObjectConverter
in ContentstackClient.

Testing

Verified with a .includeEmbeddedItems().Fetch<T>() call against a JSON RTE
field containing embedded entries and embedded assets:

• BEFORE: throws JsonSerializationException
• AFTER: deserializes successfully; uid, title, _content_type_uid and
  all custom fields are populated

Changelog

See CHANGELOG.md → Version 1.3.1