webots_ros2_driver native MacOS support

[HappyEnt]

Description
Currently webots_ros2 only builds natively on Ubuntu. Using Robostack, a partially supported ROS2 installation can also be done fairly easily directly on MacOS.
This pull requests makes some changes to the build process, so webots_ros2 also builds natively on MacOS.

Affected Packages
List of affected packages:

• webots_ros2_driver
• webots_ros2_msgs

Tasks

• Find better way to build controller library on MacOS
• Find better way to resolve dynamic library locations
• fix automatic download of webots

Additional context
This pull-request is pretty much WIP and not ready to be merged yet.
Note that for testing purposes I made changes to the reduced webots source code (the one generated by sync_controller_lib.sh) directly here in the repository.

Currently the build process for MacOS always targets an Application Bundle build, i.e., resources and dynamic libraries are expected to be prefixed with Contents/.... As webots_ros2_driver only targets linux for now, it however expects files to be in the unprefixed location. For now I solved this by only conditionally prefixing paths determined by the variable BUILD_MACOS_BUNDLE. This however requires changes in the build process of webots. Another solution would be to leave the paths as they are, and change only webots_ros2_driver to use the files under the Contents prefix.

Further note that I bumped the minimum cmake version to version 3.15 to fix a problem with findPython module during the build process. If this is undesirable I can look If I find a workaround that does not depend on a version bump.

[ygoumaz]

@HappyEnt, do you still have the will to complete this PR?