Add @kaessert as a maintainer (repo admin, OWNERS.md, CODEOWNERS) #126

Proposal to add Tobias Kässer (@kaessert) as a maintainer of this repository. They recently contributed the CVE-2026-111943 Go/distroless fixes (#123, #124) and the matching changes in function-patch-and-transform and function-sequencer.

Tasks

• Grant @kaessert the Write role on crossplane-contrib/function-cel-filter (repo settings → Collaborators and teams)
• Add @kaessert to the Maintainers list in OWNERS.md
• Add @kaessert to the fallback owners in CODEOWNERS, through a reviewed PR

Note on CODEOWNERS

Commit 9cef7f4 already added @kaessert to the fallback owners line in CODEOWNERS. It was pushed straight to main without a PR or review, so it did not follow the process in OWNERS.md ("All PRs must be approved by at least one maintainer before being merged"). That commit is being handled separately. The CODEOWNERS change should land again through a reviewed PR together with the OWNERS.md update.