Gleaner able to index sitemap.xml sources

[webb-ben]

Is your feature request related to a problem? Please describe.
At the moment, Gleaner can be configured to consume a sitemap or a CSV or sitemaps. It is not able to consume a sitemap index, which is what geoconnex serves PIDs with.

Describe the solution you'd like
I would like to be able to consume a sitemap index, and have it documented in https://github.com/internetofwater/gleaner/blob/master/configs/template/README_Configure_Template.md#configuration-of-your-source

Describe alternatives you've considered
Serving the content for gleaner to consume as a remote CSV. This is not in line with Web Best Practices.

Additional context
Add any other context or screenshots about the feature request here.