Add template signing steps

curl --request POST \
  --url https://restapi.sign.plus/v2/template/{template_id}/signing_steps \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "signing_steps": [
    {
      "recipients": [
        {
          "id": "<string>",
          "uid": "<string>",
          "name": "<string>",
          "email": "<string>",
          "role": "SIGNER"
        }
      ]
    }
  ]
}'

{
  "id": "<string>",
  "name": "<string>",
  "comment": "<string>",
  "pages": 123,
  "legality_level": "SES",
  "created_at": 123,
  "updated_at": 123,
  "expiration_delay": 123,
  "num_recipients": 123,
  "signing_steps": [
    {
      "recipients": [
        {
          "id": "<string>",
          "uid": "<string>",
          "name": "<string>",
          "email": "<string>",
          "role": "SIGNER"
        }
      ]
    }
  ],
  "documents": [
    {
      "id": "<string>",
      "name": "<string>",
      "filename": "<string>",
      "page_count": 123,
      "pages": [
        {
          "width": 123,
          "height": 123
        }
      ]
    }
  ],
  "notification": {
    "subject": "<string>",
    "message": "<string>",
    "reminder_interval": 123
  },
  "dynamic_fields": [
    "<string>"
  ]
}