Replace Wiki service find_page RPC by normal repository RPCs

What does this MR do and why?

For #357649 (closed)

In &2862 (closed), we are trying to get rid of ruby sidecar inside Gitaly. The ruby sidecar is responsible for some hard-to-migrate when we invented Gitaly long before. One of a particular feature handled by Ruby sidecar is Wiki service. When GitLab Rails requests any Wiki-related data from Gitaly, it triggers a Wiki RPC call. This call is handled by Gitaly, but it instantly forwards to a Ruby gRPC server inside Gitaly. Eventually, the call is handled by Ruby. Underlying, the whole Wiki service is handled by Gollum. This gem structures Wiki data as a Git directory. Therefore, it makes a lot of sense to let Gitaly handles those business logic via normal repository RPCs.

This MR concentrates on moving the find_page RPC. This RPC is responsible for fetching an in individual page by its title/slug. To fulfill this duty, it performs the following RPCs:

• repository#search_files_by_regexp to fetch the first matched eligible wiki pages. As a user can push anything to the wiki repository, we imply the underlying file path from the title/slug, and append the list of valid extensions.
• repository#blob_at to get the information of a page from the blob. Just because the above RPC doesn't include blob info.
• repository#repository.commit to get missing information of the page, such as author, or last updated at.
• repository#last_commit_for_path to check whether the current page is at latest version. This code path is executed only whenever users view a historical page.

I created a detailed analysis from performance perspective in this thread: #357649 (comment 1065678396). Overall, this change won't affect the performance of the system.

This replacement is behind the feature flag: #371487 (closed)

Screenshots or screen recordings

This feature does not affect users. In fact, that's the whole point of this replacement initiative. I did some manual test to ensure everything works as before.

View home page

View page history

View a historical page

View a non-ASCII title page

View file in a sub-folder

/api/v4/project/:id/wikis/:slug

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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