[#134] Add "payload_template" in "rest" and "scrape" function

[jekalmin]

Objective

Add payload_template in rest and scrape function.

Example

- spec:
    name: get_your_data
    description: Get your data
    parameters:
      type: object
      properties:
        foo:
          type: string
          description: Some interesting payload variable.
  function:
    type: rest
    method: POST
    resource: "https://YOUR_POST_API"
    payload_template: >-
      {"foo": "{{foo}}", "bar": "bar"}
    headers:
      "Content-Type": "application/json; charset=utf-8"
    value_template: '{{value_json}}'