[chore](build) Add BE code owners

[HappenLee]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: Add HappenLee and mrhhsg as code owners for the BE execution version manager and BE CMake files so changes to these critical files request the appropriate reviewers.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test
  • No need to test. This is a CODEOWNERS-only change.

• Behavior changed:

  • No.
  • Yes. Review assignments are updated for the affected paths.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?