Define CODEOWNER election process

Within S-CORE we use CODEOWNERS feature which is consistent with the Eclipse Foundation Development Process.
However, we lack a proper definition of said group and how to become a member. It's a subset of all [Committers](https://eclipse-score.github.io/process_description/main/roles/index.html#rl__committer), but being in said group might imply additional privileges and responsibilities.

Let's document that (sub-)role, and decide and document specific practices on how S-CORE grants somebody those privileges in an open, transparent, and meritocratic manner.

Example: a CODEOWNER candidate selection follows the same practices that we apply for adding new committers; that is, after a demonstration of merit, a member of the existing CODEOWNER group will use a public channel (e.g., the score-dev mailing list, a github issue etc) to nominate the individual citing the specific public examples of merit and invite other members of the CODEOWNER team to vote.

Notes:
* CODEOWNER setting restricts other Committers from contributing to certain files in certain repositories
* CODEOWNER does not grant any permissions, it only restricts
* CODEOWNER entries only make sense for committers - however as it does not add permissions, there is no harm in adding non-committers to the list.

Loosely related: #1623, #1624