Blame controller refactoring (part 2)

What does this MR do and why?

Contributes to #393082 (closed)

The refactoring process requires multiple changes. I divided the work into 3 parts to simplify the review.

• Extract Gitlab::Repository::BlameMode class

Roadmap

Merge request	Description
!116684 (merged)	Initial clean-up
!116688 (merged) 👈	Extract BlameMode
!116689 (merged)	Remove BlameService

Check the final result: !116425 (diffs)

How to set up and validate locally

1. Run Feature.enable(:blame_page_streaming) in Rails console (gdk rails c)
2. Open any blame file with more than 2000 lines. (GDK example link, 30KLOC+)
3. Click 'View entire blame' button at the bottom of the page.

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.