Add support for metadata during prompt creation

[steffendsommer]

In my CI/CD pipeline I sync prompts from code to Braintrust with a simple script that uses the Braintryst TypeScript SDK:

const project = braintrust.projects.create({ name: 'my-project' });
project.prompts.create({
    slug: 'my-prompt',
    name: 'My Prompt',
    model: 'gpt-4o',
    messages: ...
    ifExists: "replace",
});

As far as I can tell, the SDK don't support adding metadata to my prompt during creation. That would be great to be able to do that (instead of having to use the API directly).