Support for custom fields

Some feedback that was given to us from an open source developer 😸

Based on the Jira python library being used, the actual customfields themselves are just an additional key/value pair passed to the fields object: https://developer.atlassian.com/server/jira/platform/jira-rest-api-examples/#creating-an-issue-using-custom-fields

Here - https://github.com/github/ghas-jira-integration/blob/main/jiralib.py#L168 this method is where we would pass something like field={“customfield_0000”: “my_value”}

Ideally the custom field would be an arbitrary map the user passes into the github action.

Start in jiralib.py around line 168, where the Jira issue fields are passed, and trace how the GitHub Action supplies its configuration. Confirm how an arbitrary custom-field map should flow into the Jira request, then run the existing tests or action checks to verify that custom fields are included without breaking issues that omit them.

Written by the indexing model from the issue text.