Search: Add support for local development mode, staging site

### Impacted plugin

Jetpack

### What

Have Search work when in development mode.

### How

VIP would like to be able to allow developers to customize and test search while not working on a production site.

I think there are two things we can do here:
1. Add/document how to override the blog id (just a filter on the options) so that the user can specify the blog_id for a production/staging site and display the results from there. This could even be built into the UI somehow, or we could make a better filter name.
2. Add a local API to Jetpack that we fall back to when the site is in development mode. This would just return some dummy data in different formats. Ideally showing off the range of content results, but having something to start with is the important part and then we can always iterate on it.

Review Jetpack Search's development-mode behavior and the options filter first. Compare the two proposed paths—overriding a production or staging blog ID, or adding a local fallback API—and confirm the desired scope with maintainers. Done means Search can be customized and tested off production with representative results.

Written by the indexing model from the issue text.