.Net: Fix serialization of OpenAIChatMessageContent type

[SergeyMenshykh]

Motivation and Context

This PR fixes the #5264 issue that prevents auto calling of kernel functions created from prompt.

Description

1. The PR checks whether the function execution result type is ChatMessageContent or its subtype and returns the ChatMessageContent.Content property value if that's the case. Otherwise, if the check was not provided, there would be an attempt to serialize the class and because the OpenAIChatMessageContent is not and can't be registered for polymorphic serialization on the KernelContent class, the serialization would fail with the exception - "System.NotSupportedException: Runtime type 'Microsoft.SemanticKernel.Connectors.OpenAI.OpenAIChatMessageContent' is not supported by polymorphic type 'Microsoft.SemanticKernel.KernelContent'." Alternatively, instead of the type check above, the function result serializer could be configured with a custom JsonTypeInfoResolver to handle polymorphic serialization of the OpenAIChatMessageContent class. However, in this case, all the public properties - ModelId, Metadata, Content, Items, Encoding, etc. of the OpenAIChatMessageContent class would be unnecessarily serialized.
2. This PR also introduces the new ToolCallBehavior.FunctionResultSerializerOptions to manage the serialization behavior of function execution results. It can be used to provide a JsonTypeInfoResolver for the polymorphic serialization of a custom subclass of the KernelContent class.

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 😄