Add group_id in the wiki_blobs search API

What does this MR do and why?

Exposing group_id in the blobs entity if the blob is a Gitlab::Search::FoundWikiPage

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Ensure elastic search is enabled
2. Create some wiki
3. Check this URL: http://127.0.0.1:3000/api/v4/search?scope=wiki_blobs&search=#{query} Replace query with a search term from the created wiki
4. Verify that you get the group_id field in the API response.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #336100 (closed)