Bugfix: CAD attributes not available to t8_geometry_cad.cxx

[saustinp]

Describe your changes here:
Added private header to src/t8_geometry/t8_geometry_implementations/t8_geometry_cad.cxx. Previously, this file required a public header, t8_geometry_cad.hxx, for T8_CMESH_CAD_EDGE_ATTRIBUTE_KEY and T8_CMESH_CAD_FACE_ATTRIBUTE_KEY. However, the header file transitively included a private header which was not available after building, which broke compilation of programs using this feature, for example t8_features_curved_meshes.cxx.

Now, src/t8_geometry/t8_geometry_implementations/t8_geometry_cad.cxx directly includes the private header file, bypassing t8_geometry_cad.hxx for these attributes. Fixes #2220.

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