Allow specifying `RequestOptions` as part of `PromptBuilder`

[felixarntz]

While in #99 we implemented support for RequestOptions, we never enabled callers with a reasonable way to provide them. I don't think it's currently even possible to pass request options alongside a prompt that would be used for the eventual API request to the cloud provider.

Here's what I'm proposing:

• expand PromptBuilder with a usingRequestOptions(RequestOptions $requestOptions): self method
• expand ModelInterface with setRequestOptions(RequestOptions $requestOptions): void and getRequestOptions(): ?RequestOptions methods
• update the provider implementations to call getRequestOptions() when constructing their API requests

By implementing this in ModelInterface and AbstractAiProvider, this is almost not even a breaking change. It technically is, if you're not using AbstractAiProvider. But we're so early in the lifecycle of this SDK, I doubt anyone is building custom providers yet.