Add support for Google App Script (UrlFetchApp)

[rayliuca]

Added support for Google Apps Script's UrlFetchApp class as per #447

Google documentation:
https://developers.google.com/apps-script/reference/url-fetch/url-fetch-app

[rayliuca]

My implementation is based on the javascript-xhr code generator, but do note that the UrlFetchApp class is non-existent on Node.js so it is impossible to run the Newman tests. I adapted the unit tests from the xhr variant and manually tested the generated codes on GAS to make sure it has most of the common functionalities.

Sample response of the post_formData.js

[clairefro]

Would love to see this feature added!