Replace wiki#find_file RPC with Gitaly call [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

In this MR we replace the exiting wiki#find_file RPC with one of the existing Gitaly methods.

We also add the functionality to load the content or not.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have not included a changelog entry because the functionality is behind a feature flag and it's also a non facing change for the user.
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Related to #324165 (closed)