chore: codeowners (#32194)

Adding a CODEOWNERS file to so that we can require approval from
specific teams based on the changed code.

> See
https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners#codeowners-and-branch-protection

----

*By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license*

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: GZ <yuanhaoz@amazon.com>

.github/CODEOWNERS

@@ -0,0 +1,4 @@
+/packages/aws-cdk/ @aws/aws-cdk-core-team
+/packages/@aws-cdk-testing/cli-integ/ @aws/aws-cdk-core-team
+/packages/aws-cdk-lib/core/ @aws/aws-cdk-core-team
+/packages/@aws-cdk/cli-lib-alpha/ @aws/aws-cdk-core-team