lib: re-introduce Jsonable interface in json

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib/json: migrate json::static to Jsonable

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib/json: migrate json::dynamic to Jsonable

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib/json: migrate client to Jsonable

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib/json: migrate tests

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>