Fix Deserialisation of AssistantStreamEventType.THREAD_MESSAGE_DELTA

[av-runner]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Related Issue	Fix #421

Describe your change

Make the content in the Message Delta Data a list and add needed data models for the possible objects within the list.

What problem is this fixing?

The content in the MessageDeltaData data class, within the Assistant Stream Event was a MessageContent, however the response from OpenAI is a list that might contain 4 different types of objects. I have added the missing data models.

[osmanconger]

When can we expect this to be merged @aallam?

[tashilapathum]

Without this fix, deserialization would fail for this code:

AssistantStreamEventType.THREAD_MESSAGE_DELTA -> {
    val messageDelta = assistantStreamEvent.getData<MessageDelta>()
    val content = messageDelta.delta.content
}

with the error Expected JsonObject, but had JsonArray as the serialized body of com.aallam.openai.api.message.MessageContent at element: $.delta.content.

[av-runner]

Hey @aallam, are there any problems you would like me to address or can we merge this?