Feature: Checkpointing for T8codeMesh

[jmark]

This PR adds checkpointing for T8codeMesh. By this, routines like save_mesh and load_mesh are supported.

Closes #2044

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 97.88136% with 5 lines in your changes missing coverage. Please review.

Project coverage is 95.44%. Comparing base (16c6f17) to head (aca9742).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auxiliary/t8code.jl	75.00%	3 Missing ⚠️
src/callbacks_step/save_restart_dg.jl	66.67%	1 Missing ⚠️
src/meshes/t8code_mesh.jl	99.38%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1980      +/-   ##
==========================================
- Coverage   96.37%   95.44%   -0.93%     
==========================================
  Files         480      480              
  Lines       37855    38028     +173     
==========================================
- Hits        36482    36295     -187     
- Misses       1373     1733     +360     

Flag	Coverage Δ
unittests	95.44% <97.88%> (-0.93%)	⬇️

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[benegee]

Concerning the failing invalidations check, see timholy/SnoopCompile.jl#397

[jmark]

@benegee As expected, several tests fail since some routines became obsolete with the breaking t8code 3.0.0 release. We have to iron out these issues first.

I suggest we fix these problems in this PR #1939 first since the changes are minimal on Trixi side. Then merge that PR into the checkpointing PR.