feat(openai_dart)!: Add multi-modal support

[davidmigloz]

Breaking changes:

ChatCompletionMessage

• ChatCompletionMessage is now a sealed class that supports the following message types:
  • ChatCompletionMessage.system(): a system message.
  • ChatCompletionMessage.user(): a user message.
  • ChatCompletionMessage.assistant(): an assistant message.
  • ChatCompletionMessage.tool(): a tool message.
  • ChatCompletionMessage.function(): a function message.

Before:

messages: [
  ChatCompletionMessage(
    role: ChatCompletionMessageRole.system,
    content: 'You are a helpful assistant.',
  ),
  ChatCompletionMessage(
    role: ChatCompletionMessageRole.user,
    content: 'Hello!',
  ),
]

After:

messages: [
  ChatCompletionMessage.system(
    content: 'You are a helpful assistant.',
  ),
  ChatCompletionMessage.user(
    content: ChatCompletionUserMessageContent.string('Hello!'),
  ),
]

ChatCompletionMessage.user content

• ChatCompletionMessage.user accepts a ChatCompletionUserMessageContent sealed class that can be:
  • ChatCompletionUserMessageContent.string(): text content
  • ChatCompletionUserMessageContent.parts(): multi-modal content