chore: Export base Message class in OpenAI chat SDK

[kevinmingtarja]

Description

This PR would allow users to create their own message with custom role. I encountered this use case when trying to use Llama 3.1 (served using an OpenAI compatible server), which has a different set of supported roles: [system, assistant, user, ipython].

Basically, i want to be able to do:

package main

import (
	"github.com/hypermodeinc/modus/sdk/go/pkg/models"
	"github.com/hypermodeinc/modus/sdk/go/pkg/models/openai"
)

type customMessage struct {
	openai.MessageBase
}

func generate(prompt string) (string, error) {
        ...
        input, err := model.CreateInput(
		...
		&customMessage{
			MessageBase: openai.MessageBase{
				Role:    "custom",
				Content: "...",
			},
		},
	)
        ...
}

Given the increasing number of inference providers implementing the OpenAI API (to some degree), I think we might need this for other models too eventually.

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to this PR