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

tools/server/server-context.h

@@ -4,7 +4,7 @@
 #include "server-task.h"
 #include "server-queue.h"
 
-#include <nlohmann/json_fwd.hpp>
+#include "json.h"
 
 #include <cstddef>
 #include <memory>