Update and review of MAINTAINERS.MD file

[hdamker]

Problem description

To execute camaraproject/Governance#134 we need the GitHub Usernames of all Maintainers in MAINTAINERS.MD. For that purpose a new column will be added.

In addition the TSC requested to use this opportunity to review all Maintainer files if they are still current (e.g. the Maintainer still active, with the listed affiliation, etc).

Expected action

Please review the PR which will be created for this issue. Please check if all listed people are still committed, active Maintainers according to the role description. Propose needed changes.

And: Add missing GitHub Usernames.

Additional context

The PR will also add the following lines at the end of CODEOWNER file to ensure that CAMARA Admins are involved in all changes of the both files:

# Owners of the CODEOWNER and Maintainer.md files are the admins of CAMARA (to allow them to keep the teams within the CAMARA organization in sync in case of changes)
/CODEOWNERS @camaraproject/admins
/MAINTAINERS.MD @camaraproject/admins