feat: add lua_cjson module

[xiaobiaozhao]

post_json.lua

local cjson = require "cjson"
local json_encode = cjson.encode
local json_decode = cjson.decode
wrk.method = "POST"
wrk.body = json_encode({ ["key"] = "value" })
wrk.headers["Content-Type"] = "application/x-www-form-urlencoded"

1. wrk can use lua's built-in json library to do json encode&decode