…ly (microsoft#5131)

### Motivation and Context
Today, it is not possible to deserialize ChatMessageContent that has at
least one content item in the Items collection. The reason for this is
that the content items in the Items collection are referenced
polymorphically through the KernelContent abstract class. As a result,
the deserialization process fails because it cannot create an instance
of the abstract class. To solve the problem, the serialization process
should save type information when serializing the Items collection so
that the deserialization process can use this information to find the
type that was serialized and create its instance. Therefore, this PR
leverages [type
discriminators](https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/polymorphism?pivots=dotnet-8-0)
to save the type information and use it during deserialization.

### Description
This PR registers the type discriminator for the KernelContent class and
whitelists its subclasses to participate in polymorphic deserialization.
On top of that, a few content types are modified to be
serializable/deserializable.

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Co-authored-by: Dmytro Struk <13853051+dmytrostruk@users.noreply.github.com>

…ly (microsoft#5131)

### Motivation and Context
Today, it is not possible to deserialize ChatMessageContent that has at
least one content item in the Items collection. The reason for this is
that the content items in the Items collection are referenced
polymorphically through the KernelContent abstract class. As a result,
the deserialization process fails because it cannot create an instance
of the abstract class. To solve the problem, the serialization process
should save type information when serializing the Items collection so
that the deserialization process can use this information to find the
type that was serialized and create its instance. Therefore, this PR
leverages [type
discriminators](https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/polymorphism?pivots=dotnet-8-0)
to save the type information and use it during deserialization.

### Description
This PR registers the type discriminator for the KernelContent class and
whitelists its subclasses to participate in polymorphic deserialization.
On top of that, a few content types are modified to be
serializable/deserializable.

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Co-authored-by: Dmytro Struk <13853051+dmytrostruk@users.noreply.github.com>