Implement reset interface in the physics system

[mjcarroll]

This builds upon the reset interface for systems introduced in #1249 by implementing the interface for the Physics system.

Upon receiving a reset command, this allows the physics plugin to rewind to the initial state of the world without removing and re-adding all of the entities.

[codecov]

Codecov Report

Merging #1327 (32d21f5) into main (e399431) will increase coverage by 27.40%.
The diff coverage is 58.49%.

❗ Current head 32d21f5 differs from pull request most recent head fbf6d0e. Consider uploading reports for the commit fbf6d0e to get more accurate results

@@             Coverage Diff             @@
##             main    #1327       +/-   ##
===========================================
+ Coverage   35.01%   62.42%   +27.40%     
===========================================
  Files          44      317      +273     
  Lines        2356    24562    +22206     
===========================================
+ Hits          825    15333    +14508     
- Misses       1531     9229     +7698     

Impacted Files	Coverage Δ
src/systems/physics/Physics.hh	100.00% <ø> (ø)
src/systems/physics/Physics.cc	66.00% <55.10%> (ø)
src/systems/physics/CanonicalLinkModelTracker.hh	100.00% <100.00%> (ø)
...ins/component_inspector/moc_ComponentInspector.cpp
src/gui/plugins/plot_3d/moc_Plot3D.cpp
src/gui/plugins/entity_tree/qrc_EntityTree.cpp
...c/gui/plugins/scene_manager/qrc_GzSceneManager.cpp
src/gui/plugins/spawn/moc_Spawn.cpp
...n-gazebo7-gui_autogen/EWIEGA46WW/moc_GuiRunner.cpp
...gui/plugins/select_entities/qrc_SelectEntities.cpp
... and 354 more

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[chapulina]

It would be nice to add a full description for this PR. It only refers to another PR that doesn't explain anything about this one.

Are there plans to support models that are created or removed during the simulation? This limitation should be documented somewhere.

The PR looks good to me besides the comments below 👍🏽

[codecov]

Codecov Report

Merging #1327 (e324b12) into main (9370793) will not change coverage.
The diff coverage is n/a.

❗ Current head e324b12 differs from pull request most recent head b5f6bc8. Consider uploading reports for the commit b5f6bc8 to get more accurate results

@@           Coverage Diff           @@
##             main    #1327   +/-   ##
=======================================
  Coverage   35.01%   35.01%           
=======================================
  Files          44       44           
  Lines        2356     2356           
=======================================
  Hits          825      825           
  Misses       1531     1531           

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[chapulina]

I believe all the CI failures are pre-existing