feat: Support converting json data into form for submission

[mo2g]

Supports converting the following json data into application/x-www-form-urlencoded type before submitting it

###
# @jsonToForm
POST https://httpbin.org/post 

{
  "resp_desc" : "交易成功[000]",
  "resp_code" : "00000000",
  "sign" : "123+456",
  "resp_data" : "{"acct_date":"20260305"}",
}

can be converted to the following curl request

curl --request POST \
  --url https://httpbin.org/post  \
  --header 'Content-Type: application/x-www-form-urlencoded' \
  --header 'User-Agent: vscode-restclient' \
  --data 'resp_desc=交易成功[000]' \
  --data resp_code=00000000 \
  --data 'sign=123+456' \
  --data 'resp_data={"acct_date":"20260305"}'