Support Thread and Message for Assistant v2

[ZhihanWei]

Is your feature request related to a problem? Please describe.
OpenAI has released their new version of assistant(v2) and the old version of assistant(v1) will be deprecated by the end of 2024. There are some incompatibilities between the two version in terms of API request body.
For example, MessageRequest in v1

type MessageRequest struct {
	Role     string         `json:"role"`
	Content  string         `json:"content"`
	FileIds  []string       `json:"file_ids,omitempty"` //nolint:revive // backwards-compatibility
	Metadata map[string]any `json:"metadata,omitempty"`
}

MessageRequest in v2, which introduced attachments

type MessageRequest struct {
	Role        string         `json:"role"`
	Content     string         `json:"content"`
	Attachments []Attachment   `json:"attachments,omitempty"`
	Metadata    map[string]any `json:"metadata,omitempty"`
}

We let the user configure the assistant version in the client configuration but the new assistant version will cause trouble at runtime when making calls to Thread and Message.

Describe the solution you'd like
Support both assistant v1 and v2.

Additional context
There are different implementations I can think of that could make the enhancement, but most likely the compatibility will be checked at runtime. Would be good to see an implementation to support both assistant versions and check errors at compile time while maintaining the configured assistantVersion.

[Trojan295]

There are more changes, because the Content can be now a string, but also an array of objects. https://platform.openai.com/docs/api-reference/messages/createMessage