Add client-side analytics callback

[yujonglee]

One example is that if user actually click the result after query is performed.

[yujonglee]

• https://developer.matomo.org/guides/tracking-javascript-guide#internal-search-tracking
• https://www.algolia.com/doc/ui-libraries/autocomplete/core-concepts/plugins/

[yujonglee]

1. We need /event route
2. Each search and ask need unique ID (in meta, like we do with session_id.)
3. We need to hit /event route from the client side

[yujonglee]

tracking_id of chunk is not really useful since we keep refetch from source.
Thing like URL is much better.