Azure-kinect-sensor-sdk: Please add the CMake file for offline_processor

[legacydev]

The source does not include the CMake file for the offline_processor. Can this be added?

[wes-b]

Our goal with #864 was to get enough of a CMake system in place to enable the community to help build out the samples.

We would be happy to answer questions if issues are encountered.

[legacydev]

I've added a comment to #864 as I am unable to build it. Currently haven't had a chance to go through moving this particular project to CMake, yet.

[KankshaZ]

The following steps helped me build offline_processor:

1. Added json.hpp to sample_helper_includes folder
2. Appended the line add_subdirectory(offline_processor) to CMakeLists.txt in body-tracking-samples
3. Added CMakeLists.txt to offline_processer which contains

add_executable(offline_processor main.cpp)
target_include_directories(offline_processor PRIVATE ../sample_helper_includes)
target_link_libraries(offline_processor PRIVATE 
    k4a
    k4abt
    k4arecord
    window_controller_3d::window_controller_3d
    glfw::glfw
    )

Should I make a pull request and add these files?