CI: Add GitHub label for extension related changes

[SMoraisAnsys]

Description

Add a label dedicated to extensions. This is in preparation for the effort of splitting the extensions into a separate repo and handling work that might be performed in pyaedt during the OSS process.

Issue linked

None

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate tests (unit, integration, system).
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved by the PR if any.
• I have agreed with the Contributor License Agreement (CLA).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.32%. Comparing base (b8d00cf) to head (daee216).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6814   +/-   ##
=======================================
  Coverage   83.31%   83.32%           
=======================================
  Files         246      246           
  Lines       77423    77423           
=======================================
+ Hits        64505    64509    +4     
+ Misses      12918    12914    -4     

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