common: add json.h abstraction (#27511)
* add common/json
* migrate common
* adapt jinja
* migrate server
* big wip
* migrate tests
* wip
* revert some excessive changes
* wip
* wip 2
* revert redundant changes
* fix server crash
* various fixes
* fix ci
* harden a bit
* clean up
* rm json-shim
* add some comments
* rm redundant decl
