Optional maintainers field in config.yaml, to notify framework authors

In #9810 the question came up whether to change other frameworks' entries. I did not want to do it: the entry is not mine, and the author is the only one who can say if the change is fair for his framework.

Today there is no way to reach the author of an entry. CODEOWNERS is only @waghanza, and the github field in config.yaml points at the framework repository, not at a person.

Proposal: an optional field in config.yaml

framework:
  github: nigrosimone/fulmine.js
  maintainers:
    - nigrosimone

With it, a workflow can mention the listed handles when a PR touches their directory, and they can also be pinged for other things that matter to them, like a change of the rules or of the load generator. Who does not add the field is not notified, so nothing changes for the existing entries.

Related to #9090, which asked for the results by email. Handles on GitHub are simpler: no addresses to store, and the notification arrives where the discussion already is.