When the above cmake option is turned ON, via the below command
```cmake
ccmake .. -DAF_BUILD_OFFLINE:BOOL=ON
```
FetchContent will look for dependencies under build tree's extern
folder and will not attempt to download any of them.
By default this option is turned OFF
Description
Changed all the following submodules to be fetched via CMake's FetchContent command. Unfortunately, this command is made available only in 3.11.4, thus we copied the required files to our repository's CMakeModules folder.
Also, marked cmake variables with FETCHCONTENT prefix as advanced
Changes to Users
None to downstream users. Repository cloning and starting the build process is expected to become slightly easier.
Checklist