Enhance jira crawler with custom metadata

[eskibars]

With almost every Jira install, there's some custom metadata. For example, you might have customfield_10008 which may align to the GitHub status of a related GitHub PR that you added from the GitHub/Jira integration. Or a support manager might have added a field like "root cause of problem" holding the root cause of some bug and Jira might decide that's customfield_10041. The value of these fields could be very useful for either metadata or search purposes to load into Vectara. It would be great to give users the power to provide a custom mapping in config that could then be pulled in to the crawler