CODEOWNERS: Add files which need Gitaly's approval

What does this MR do?

We've seen grown some inefficient access patterns in these files. In an attempt to overcome more of these get added, make a few Gitaly people --who are familiar with the Rails side of things-- code-owner of these files so they will be asked for review when these files are touched.

Relates-to: &5717 (closed)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.