.Net: Support polymorphic serialization of ChatMessageContent class and its derivatives

[SergeyMenshykh]

Motivation, Context and Description

Today, when serializing chat history, the ChatMessageContent type or its derivatives, like OpenAIChatMessageContent, are serialized as the ChatMessageContent type because the type is used as the chat history element type and the JSON serializer uses this type's public contract for serialization.

However, when attempting serialization of instances of either the ChatMessageContent or OpenAIChatMessageContent type that are declared as KernelContent or object type, the serialization fails with the System.NotSupportedException: Runtime type '{OpenAI}ChatMessageContent' is not supported by polymorphic type 'Microsoft.SemanticKernel.KernelContent' exception. The reason for this exception is that neither of these types is registered for polymorphic serialization.

This PR registers ChatMessageContent type for polymorphic serialization to allow serialization of the type instances declared as of KernelContent or object types:

KernelContent content = new ChatMessageContent(...);

// Now it's possible to serialize the content variable of KernelContent type that holds reference to an instance of the ChatMessageContent type as ChatMessageContent type.
var json = JsonSerializer.Serialize(content); 

Additionally, it enables serialization of unknow in advance and external derivatives of ChatMessageContent type like OpenAIChatMessageContent. These types are serialized using contract of nearest ancestor which is ChatMessageContent by default. To change this behavior and register the unknown type for polymorphic serialization use the contract model - Configure polymorphism with the contract model

KernelContent content = new UnknownChatMessageContent(...);

// The content variable will be serialized using the ChatMessageContent type contract.
var json = JsonSerializer.Serialize(content);

private class UnknownChatMessageContent : ChatMessageContent{}

Closes: #7478

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄