Permit building python bindings separately from main library

[scpeters]

🎉 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)
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