CMakeLists.txt improvements for packaging support

[valgur]

Hi! This PR proposes a few minor tweaks to the CMakeLists.txt to simplify potential packaging as a Conan (a recipe draft) or Vcpkg package.

The changes:

• Add basic install() commands for the standalone C++ library.
• Make CMAKE_POSITION_INDEPENDENT_CODE configurable.
• Allow SHARED builds.
• Set cxx_std_17 property instead of set(CMAKE_CXX_STANDARD 17) to make it easier to override the standard, if necessary.
• Make USING_ROS2 an explicit option.
• Add an EXTERNAL_FASTCDR option.
• Add missing target_link_libraries() for rapidjson.

[valgur]

Also, for external use, would a rosx_introspection::rosx_introspection CMake target be ok or would you prefer a non-namespaced rosx_introspection?

[facontidavide]

looks good to me. Thanks for the help