Add explicit owner group support for Teams and Channels

[Sup3rlativ3]

Summary

This change adds explicit owner-group mapping support for Team and Private Channel membership sync.

What changed

• added OwnerGroups support to the team/channel mapping config template
• kept backward compatibility for existing Groups[].Role = "Owner" mappings
• promoted mapped owners before removal/demotion checks so last-owner operations do not fail unnecessarily
• added guarded handling for Graph "last owner" failures during member removal and owner-role removal
• updated README documentation for OwnerGroups, Group mappings, and owner-role behavior

Validation

• I have been using this for a few clients and it is working perfectly.

[Sekers]

Thank you @Sup3rlativ3 . I will do a code review when I next have the chance. I had some uncommitted updates to resolve as well.

[Sup3rlativ3]

If you let me know once your changes are all done, I'll review mine and make them fit.