Permit building python bindings separately from main library (backport #554)

[mergify]

🎉 New feature

Part of osrf/homebrew-simulation#2834, related to gazebosim/gz-math#636

Summary

This allows the python/CMakeLists.txt file to be built as a top-level cmake project against an external gz-transport library.

• The first two commits (e6e6c6f and 87c80e7) are patches from @traversaro used in conda.
• The third commit (f11c332) replaces the GZ_LIB_INSTALL_DIR cmake variable with CMAKE_INSTALL_LIBDIR (see GzPackaging.cmake:111) since GZ_LIB_INSTALL_DIR is not defined with the minimal cmake project added in e6e6c6f.
• In b9f4d04, the find logic for pybind11 is moved to the python/CMakeLists.txt file.

Test it

1. Invoke cmake on the root CMakeLists.txt with using -DSKIP_PYBIND11=ON to build and install the libsdformat library without python bindings.
2. Invoke cmake on python/CMakeLists.txt with -DPython_EXECUTABLE=/path/to/python to build and install python bindings for a given python version
3. Repeat 2 with for each desired version of python

TODO: update homebrew formula using this branch in a draft PR

Checklist

• Signed all commits for DCO
• 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)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

---

This is an automatic backport of pull request #554 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 250e95f has failed:

On branch mergify/bp/gz-transport13/pr-554
Your branch is up to date with 'origin/gz-transport13'.

You are currently cherry-picking commit 250e95f0.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CMakeLists.txt
	modified:   tutorials/02_installation.md

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   python/CMakeLists.txt

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.16%. Comparing base (eac2e69) to head (0f9b052).
Report is 25 commits behind head on gz-transport13.

Additional details and impacted files

@@                Coverage Diff                 @@
##           gz-transport13     #555      +/-   ##
==================================================
- Coverage           87.69%   87.16%   -0.53%     
==================================================
  Files                  59       58       -1     
  Lines                5704     5610      -94     
==================================================
- Hits                 5002     4890     -112     
- Misses                702      720      +18     

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[scpeters]

squashed 9d14160 and 9493198 into the base commit

[scpeters]

tested in osrf/homebrew-simulation#2875

• https://build.osrfoundation.org/view/gz-harmonic/job/gz_transport13-install_bottle-homebrew-amd64/340/