Add setApiKey method

[badasswp]

Describe the feature or improvement you're requesting

At the moment, it is difficult to effectively mock our OpenAi class in unit tests because we are currently forced to pass in the API key as an argument during instantiation like so:

new OpenAi(getenv('OPENAI_API_KEY'));

I'm proposing a new setApiKey method that will enable us do this anytime so that we are not restricted to the constructor. In this way, we can create and pass our objects via dependency injection like so:

interface AiClient
{
    public function chat($payload): void;
}

public function getAiClient(AiClient $aiClient)
{
    return $aiClient;
}

public function getOpenAiClient()
{
    $this->getAiClient(new OpenAi())->setApiKey($keys);
}

Additional context

In this way, we should be able to easily mock our OpenAi class for use in unit tests like so:

public function testOpenAiDoesSomething()
{
    $openAiMock = Mockery::mock(OpenAi::class)->makePartial();
    // write your other test logic....
}