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-task.h

@@ -11,7 +11,6 @@
 // TODO: prevent including the whole server-common.h as we only use server_tokens
 #include "server-common.h"
 
-using json = nlohmann::ordered_json;
 
 enum server_task_type {
     SERVER_TASK_TYPE_COMPLETION,