Enhance MettleCI Workbench ServiceNow integration to allow credential specification

We understand that ServiceNow OAuth authentication is a three-way process, Workbench, after initial Oauth handshaking, submits a third party credential to be responsible for the actual queries. We also understand that currently, the third party credential supplied is the user currently logged in to Workbench (just as it is to git for commits, and to DataStage for DS operations)  After authentication, queries use the Service Now API via https GET requests.

 At Huntington, ServiceNow operates under a policy that only limited accounts have access to the API interface, it is not granted to general user populations, but rather only to role accounts, with very limited exceptions, which are sunsetted (so no new exceptions will be granted)

So Workbench needs to supply one of these role account credentials, but does not currently have a way to retain these in the ServiceNow configuration.  The current temporary configuration where specific users are granted access to the API is not allowed by policy, it was only enabled temporarily to aid with debugging

The enhancement request is to add the ability for the ServiceNow configuration to  retain a user id and secret to be used for this purpose, and that the UI be enhanced to let admin users manipulate these, with appropriate masking, as is done elsewhere in Workbench configuration. Allowing the secret part of these credentials to be kept in the .secrets directory (via the ${file ...} construct) adds convenience and avoids having plaintext credentials in the issue management yaml file, but that's optional