Add CMake macro for generating pybind11 bindings

[Bi0T1N]

Desired behavior

Right now there is some code duplication in ign-gazebo and ign-math for finding pybind11. I assume the goal is to provide Python interfaces also for other packages in the future, thus it makes sense to have a common macro for it.

Implementation suggestion

Since PythonLibs is deprecated, the snippet should also use FindPython3 which will be provided with PR #213 .
It might be useful to extend the new IgnPython to provide several arguments as input in order to reuse the existing code but this might require some additional effort than creating an independent IgnPythonBindings macro.

[j-rivero]

Right now there is some code duplication in ign-gazebo and ign-math for finding pybind11. I assume the goal is to provide Python interfaces also for other packages in the future, thus it makes sense to have a common macro for it.

While packaging new versions of ign-gazebo/ign-math I had the same thought. It would be great if we can de-duplicate the code and use a unify macro for it.

It might be useful to extend the new IgnPython to provide several arguments as input in order to reuse the existing code but this might require some additional effort than creating an independent IgnPythonBindings macro.

Currently IgnPython works as a CMake Module working just by including it. We can add a macro in the same file and avoid the creation of a new file in the repository. That could make sense since there are probably no use cases where someone needs to use the python bindings macro but don't want to check for a python3 installation.

[Bi0T1N]

It might be useful to extend the new IgnPython to provide several arguments as input in order to reuse the existing code but this might require some additional effort than creating an independent IgnPythonBindings macro.

Currently IgnPython works as a CMake Module working just by including it. We can add a macro in the same file and avoid the creation of a new file in the repository. That could make sense since there are probably no use cases where someone needs to use the python bindings macro but don't want to check for a python3 installation.

I made the needed changes for pybind11 in gazebosim/gz-math#402 to fix the related issue.
There is a small difference, we need find_package(Python3 QUIET COMPONENTS Interpreter Development) instead of find_package(Python3 QUIET COMPONENTS Interpreter) = find_package(Python3 QUIET) which is used in ign-cmake. We could use a macro which also searches for Development but in my opinion it'd be nicer to use a different include statement.
macro:

include(IgnPython)
search_pybind11_library() # does a find_package(Python3) twice, once without Development and once with (+ there are even more options like numpy/different developments)

separate module:

include(IgnPythonBindings)

I don't know but might it be possible to do include(IgnPython *my_arguments_list*)? Then the items to be searched for can directly be declared within the include command.

However, for similarity between different Python 'includes' I think it would make sense to have a common way on how to use/find Python3 - a single macro for each case of finding Python3, a macro with several arguments to specify the needs or separate and specific include modules.