Writing prompts: fetch prompt from API by id

[creativecoder]

What

Update the blogging-prompts API to retrieve prompts by ID.

Why

With writing prompt notifications, add a query param ?answer_prompt={prompt_id} when starting a new post in the editor to embed that prompt in the post and mark the post as an answer to the prompt.

However, the current prompt API only accepts date based queries, making it difficult to align a specific prompt ID with the results from the API.

How

• Add an additional endpoint: /sites/{site}/blogging-prompts/{prompt_id} that returns a prompt with that ID.
• Update Jetpack extension to use new endpoint for fetching (and caching) prompts

[creativecoder]

Initial diff for API endpoint at D97676-code.

[creativecoder]

And here's the Jetpack PR for the editor extension changes: Automattic/jetpack#28332

[creativecoder]

This is addressed in Automattic/jetpack#29433