chore: Configure CODEOWNERS to automate review requests

[valfirst]

Change list

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners:

Code owners are automatically requested for review when someone opens a pull request that modifies code that they own.

Types of changes

What types of changes are you proposing/introducing to Java client?

• No changes in production code.
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)