For ServiceNow Integration, support client_credentials Grant type

Currently for the version 1.0.69 of the ServiceNow Integration, the OAuth authentication requires a Refresh Token and uses the corresponding Grant Type of “Refresh Token”.
However, acquiring a Refresh Token in ServiceNow requires an “out-of-band” authentication, as described in this post:
https://www.servicenow.com/community/developer-forum/how-to-generate-refresh-token/m-p/1568368

The authentication flow is further documented here:
https://www.servicenow.com/docs/bundle/zurich-platform-security/page/integrate/machine-identity/task/third-party-token-worflow-for-user-accounts.html

Also important to highlight that, when a Refresh Token is expired, API calls using it will return an HTML document instead of a JSON reply.

So the recommended integration for ServiceNow is to use the client_credentials Grant type, following this document from ServiceNow: 
https://www.servicenow.com/docs/bundle/zurich-platform-security/page/integrate/machine-identity/task/third-party-token-workflow-for-service-accounts.html

This relies on the creation of a ServiceNow service account user:
https://www.servicenow.com/docs/bundle/zurich-platform-security/page/integrate/machine-identity/task/configure-a-third-party-id-token.html

This approach requires only the client_id and client_secret, and no refresh_token and changes the grant_type from refresh_token to client_credentials.

To progress with our proof of concept, I have modified the ServiceNow integration code:

•  authschemas/ServiceNow/ServiceNowOAuth.json
  • Added grant_type as a required field with ["refresh_token", "client_credentials"]
  • Changed refresh_token from required to optional.
• Javascript/ServiceNow/authenticate.js
  • Changed grant_type to consume the field created in the authschemas
  • Injects Refresh Token if set
     

For reference, I have attached the modified code to this idea. 

We would need the ServiceNow Integration code to officially support the client_credentials before we progress to an official deployment within the client infrastructure.