Provide the possibility to include headers using target

[igorsnunes]

Description

In order to use the target_link_directories on an external project, an extra statement had to be added to the target_include_directories, as shown on the diff. If that is not done, the restclient_cppTarges.cmake file will not contain the INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include" statement, under set_target_properties. As a consequence, it won't be possible to include the headers on my project using:

target_include_directories(${LibName} PRIVATE restclient-cpp)

• CI passes
• Description of proposed change
• Documentation (README, code doc blocks, etc) is updated
• Existing issue is referenced if there is one
• Unit tests for the proposed change