What

• Fixes Provide CMake setup for using the C++ sdk with pre-built rerun_c #3759

Change CMakeLists.txt in rerun_cpp to work standalone with add_directory.
When used in this fashion it expects the RERUN_C_LIB variable to be set. The default for it points to a local lib folder with appropriately named libs.
This in turn gives rise to a very trivial bundling strategy:

• copy src
• copy CMakeLists.txt
• copy libs

This is done in the new build_and_upload_rerun_cpp script which on top of the above also takes care of testing the resulting artifact by running the now also compatible minimal example outside of the repo.

Another smaller necessary change is that the rerun.h C header is now copied into the rerun_cpp sdk with a patched version number in the comment. This is primarily so to keep the above process super simple and shave of an additional include directory.

Draft until PR ci confirmed to be working.
(PR checks only the linux artifact, main ci checks the full artifact and puts it on the Development Release notes)

With this change, the minimal possible cmake script for a project using Rerun is:

project(hello_rerun)

include(FetchContent)
FetchContent_Declare(rerun_sdk URL https://build.rerun.io/commit/f4e5ad6/rerun_cpp.zip DOWNLOAD_EXTRACT_TIMESTAMP YES)
FetchContent_MakeAvailable(rerun_sdk)

add_executable(hello_rerun main.cpp)
set_property(TARGET hello_rerun PROPERTY CXX_STANDARD 20) # Rerun requires C++17, but you can of course use an ever newer version.
target_link_libraries(hello_rerun PRIVATE rerun_sdk)

⚠️ this STILL requires libarrow to be installed somewhere where cmake can find it ⚠️

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

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