This change starts with Jsonnet as the canonical, ready-made sandboxed library
example. Follow-up changes should similarly migrate the OSS Internship
sandboxes.
- Add an `add_sapi_subdirectory()` which sets up source and binary directories
correctly when consuming SAPI as sub-project
- Restructure the Jsonnet `CMakeLists.txt` and simplify header inclusions
- Update the Jsonnet README file
PiperOrigin-RevId: 425818479
Change-Id: Iba9e83201863b4ad8a91914397b310d9d4230423
