Allow using custom or newly released Grok models instead of restricting to enum values

[eslamabdallah74]

Hey, thanks for the great package.

I noticed something that makes it hard to use new Grok models as they come out.
When creating ChatOptions, the model argument only accepts values from the package’s enum. This becomes limiting if I want to use a model that isn’t included in that enum or if Grok releases a new model that the package hasn’t added yet.

Example:

$options = new ChatOptions(
    model: $grokModel,
    temperature: $temperature,
    stream: $stream
);

Right now, $grokModel must be one of the enum values. This prevents passing a string for a custom model.

It would be helpful if the package allowed either:

free-form string values for the model, or

an additional method to override/extend available model names.

This would make the package more flexible and future-proof.

Thanks!