Update CODEOWNERS to include engineers from both product groups for critical path files

[lukeheath]

Goal

User story
As a Fleet contributor,
I want to know which files require special consideration or review from other engineers
so that I can make sure not to merge code that could break other workflows.

Context

• Requestor(s): @lukeheath
• Product designer: @lukeheath

Changes

Engineering

• Define critical files that require a special review process.
• Define engineers from each product group that will be assigned reviewers.
• Submit PR with changes to codeowners file to implement review requirement.
• Document the special review process in the Engineering handbook.