Extract EntityStorage to a shared location so it can be used by other physics engines (backport #894)

[mergify]

Summary

Split out of #811

The EntityStorage class is useful for tracking mappings between gz-physics entities and the underlying physics engine. This PR moves it out of the dartsim component and puts it in the include/gz/physics/details directory so that it can be used by all physics engine plugins. I put it in details since it is meant to be used internally by the engine plugins.

This also fixes a couple of SDF warnings.

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the feature
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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• Was GenAI used to generate this PR? If so, make sure to add "Generated-by" to your commits. (See this policy for more info.)

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---

This is an automatic backport of pull request #894 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of a9ea56c has failed:

On branch mergify/bp/gz-physics7/pr-894
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Changes to be committed:
	new file:   include/gz/physics/detail/EntityStorage.hh
	modified:   test/common_test/simulation_features.cc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   dartsim/src/Base.hh
	both modified:   dartsim/src/SDFFeatures_TEST.cc
	both modified:   test/common_test/worlds/test.world

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