Serialize the dynamic core boundary temperature plugin

[Francyrad]

This reworks the old #6918 branch on top of current main and keeps the patch limited to the serialization hooks requested in review.

The PR adds serialize(), save(), and load() to the dynamic core boundary temperature plugin. It serializes the dynamic core state plus the small amount of runtime state needed for checkpoint/restart to restore the plugin object itself, including the OES data table when present. It does not add extra restart-path logic in update().

Validation run locally:

• git diff --check
• cmake --build build-debug --target aspect -j20
• cmake --build build-debug-nounity --target aspect -j20
• manual checkpoint/resume smoke test for dynamic_core: resumed run continues with Dynamic core data updated
• ASPECT_SOURCE_DIR=/home/francyrad/Documenti/aspect_debug/aspect_pr_6918_work ctest --output-on-failure -R dynamic_core: dynamic_core and dynamic_core_fully_solid pass; dynamic_core_fully_molten only differs in the last digit of two statistics time values on this machine

Refs #6744. Follow-up to #6918.

[Francyrad]

Note on the current no-unity-build CI failure: it fails during CMake configuration before compiling this patch because the runner installs/finds deal.II 9.3.2, while current ASPECT requires a newer deal.II.

Relevant log line:

*** Could not find a suitably recent version of deal.II. ***

I also tested this locally with deal.II 9.6.1 using ASPECT_UNITY_BUILD=OFF, and the no-unity build passed.

[tjhei]

it fails during CMake configuration before compiling this patch because the runner installs/finds deal.II 9.3.2,

The automated testers are currently failing due to issues with Ubuntu servers.

[Francyrad]

Now that #6924 has been merged, is there anything else I should update in this PR?

As far as I understand, this PR is now limited to the serialization hooks for the dynamic core boundary temperature plugin (serialize(), save(), and load()), while the CoreData initialization issue has been handled separately in #6924.

I can rebase/retest this branch on current main if useful.