Make it possible to provide a custom `download` implementation

[Nemikolh]

Feature Description

Hey! Thanks a ton for the super nice package! 🙌

I was wondering if the ability to customize downloadImplementations could be exposed to streamText via an option.

It seems that the feature is almost there! 👀 I noticed that convertToLanguageModelPrompt can accept a custom downloadImplementation:

ai/packages/ai/core/prompt/convert-to-language-model-prompt.ts

Lines 21 to 26 in c738971

	export async function convertToLanguageModelPrompt({
	prompt,
	modelSupportsImageUrls = true,
	modelSupportsUrl = () => false,
	downloadImplementation = download,
	}: {

So essentially, have this

ai/packages/ai/core/generate-text/stream-text.ts

Lines 431 to 439 in c738971

	const promptMessages = await convertToLanguageModelPrompt({
	prompt: {
	type: promptFormat,
	system: initialPrompt.system,
	messages: stepInputMessages,
	},
	modelSupportsImageUrls: model.supportsImageUrls,
	modelSupportsUrl: model.supportsUrl,
	});

also provide that option.

Would it be something that we could add to support?

Use Cases

This would make it possible to:

• Get images from somewhere else, not necessarily by making a fetch call. E.g., from an R2 bucket in Cloudflare
• Transform the URL

Additional context

No response

[lgrammel]

Looking at the code, I would prefer to just make the fetch function that is used for the download configurable. Would that work for your use case?