Allow additional ServiceNow fields

[mitchell-es]

We have a need in our environment to set an additional field for the created incidents. It happens to be a custom field so I don't expect elastalert2 would ever support it directly so I took a page from the Jira alerter and allowed specifiying additional fields with a 'servicenow_arg_' prefix. I've got a branch with the changes and it passes the unit tests and has updated docs. This would be my first time submitting a PR to this project so I figured I'd check in first see if anyone had thoughts on it.

Specifically, is the chosen prefix OK or is there something that would work better? Also, the new syntax would actually allow removing most of the existing options for the rule as they can all be specified using the new syntax. But that would either break existing configurations or require a lot of extra code to manage both the 'new' and 'old' style so I did not invest any time going down that path. I'm happy to do that work but only if the community would find it valuable.

Anyway, the branch can be seen at https://github.com/mitchell-es/elastalert2/tree/allow_additional_servicenow_fields Feedback is welcome. Thanks!

[jertel]

Yes, please submit a PR with these changes. I prefer to avoid breaking changes and there's no harm in having the existing, explicit fields available for users to make use of. The servicenow_arg_u_... syntax could be confusing for newer users anyway.

Please change these lines:

            if snow_field.startswith('servicenow_arg_'):
                payload[snow_field[15:]] = value

to:

            snow_arg_prefix = 'servicenow_arg_'
            if snow_field.startswith(snow_arg_prefix):
                payload[snow_field[len(snow_arg_prefix):]] = value

Also review the contribution guidelines and make sure all the requirements are meant. The PR will also display a list of checkboxes you'll need to check off before it can be merged.

[mitchell-es]

Thanks for the feedback! I made the improvements you suggested and got the PR submitted. I think I met all the guidelines but let me know if I missed anything. Thanks!