Consider CODEOWNERS to improve maintainer workload

Based on a [previous discussion] about merging the openwrt-routing feed into this repository, I want to suggest a new workflow possibly in favour of everyone.

The main concern of routing maintainer seem to be the overwhelming number of notifications from the packages repository. That's true as multiple hundreds of packages are maintained while the routing repository only contains a few dozens.

To improve that I'd like to consider the usage of a CODEOWNERS file. It contains patch within a repository and links them to individuals or teams. This concept exists both for GitHub and GitLab (in case OpenWrt ever considers moving fully to a self hosted GitLab instance).

The file contains lines like the following, marking me as a maintainer:

/utils/syncthing/ mail@aparcar.org

Whenever someone tries to change a file in my maintained package, I'll get a notification. To make thins more secure, it is possible to lock down the merging of pull requests so that it requires a approval from the code-owner to merge anything.

The CODEOWNERS file is only editable by admins and repository owners.

Using a CODEOWNERS file essentially allows to give more people commit access to packages.git, as they can only modify their maintained packages.

Additionally it is possible to set the number of required review to two, meaning even a codeowner can't upgrade their owned packages and only are able to merge if a second, likely admin reviewer approves the changes.

It's possible to create a CODEOWNERS file based on the PKG_MAINTAINER.

Read CONTRIBUTING.md and the linked previous discussion first, then review how PKG_MAINTAINER entries are represented across the repository. Compare the GitHub and GitLab CODEOWNERS documentation; the issue is complete only when maintainers agree on the workflow, ownership source, review requirements, and how the resulting configuration will be maintained.