Add support for ServiceNow REST API

[rguldener]

ServiceNow uses OAuth 2 for their REST API: https://docs.servicenow.com/bundle/vancouver-platform-security/page/administer/security/concept/c_OAuthApplications.html

We got some requests for it, would be great to add support if it does not require platform changes (I did not check in-depth yet).
We only need to implement the authorization_code grant for now.

[hassan254-prog]

Hi @rguldener , for a simpler understanding, one can have a look at the following thread from ServiceNow Knowledge base.