feat(js/ai/prompt): added prepare fn option to definePrompt

[pavelgj]

(from discord)

const kbRetriever = defineFirestoreRetriever(const retriever = defineFirestoreRetriever(ai, {
  name: 'exampleRetriever',
  firestore,
});

const ragPrompt = ai.definePrompt({
  prepare: async (input, {context}) => ({
    context: {...context, userInfo: await fetchUserInfo(context.auth.uid)},
    docs: await kbRetriever({query: input.query}),
  })
})

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[mbleigh]

Suggested change

	* A function that can be passes to the prompt that can produce additional propmpt render options
	* like context, docs, or any other propmpt option really:
	* A function that can validate, fetch, and populate additional information necessary for the prompt to execute.
	* The `prepare` function can be used to retrieve documents, add personalized user information to the context,
	* or generally perform any other work necessary to configure the prompt based on runtime input.

[mbleigh]

Maybe we should mark this with @beta in docs and add an assertUnstable to the code that handles it. The main concern I have about marking this stable is that I wonder if it should be allowed to manipulate the input, not just context, and that would cause weirdness with types.