Improvement: Move CAD management to t8_cad_handle with cmesh ownership

[Lena-Richter]

Closes #2218

Describe your changes here:

Changing t8_cad to t8_cad_handle for a more explicit CAD shape management. The cmesh now owns the CAD handle and uses a reference counter to manage it.
Also, the t8_geom prefix was removed for everything related to CAD shape management for cleanup. The mesh reader and geometry implementations were updated to support this new handle system.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.25%. Comparing base (322342b) to head (cb0c420).
⚠️ Report is 207 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2093      +/-   ##
==========================================
+ Coverage   77.97%   78.25%   +0.27%     
==========================================
  Files         113      114       +1     
  Lines       19106    19050      -56     
==========================================
+ Hits        14898    14907       +9     
+ Misses       4208     4143      -65     

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