text: Title: Generic XML adapter
  Body: **Is your feature request related to a problem? Please describe.**
  Currently, I am not able to query XML files using Shillelagh. I have conducted Google searches, searches of project documentation, and a search of the Apache Superset Slack but have not found an off-the-shelf solution.
  
  **Describe the solution you'd like**
  I would like to provide source data for an Apache Superset chart from an XML file not on a local file system but available from a URL.
  
  **Describe alternatives you've considered**
  The only other thing I can think of would be to read the XML file into a Pandas DataFrame and query that (cf. #388 and related Slack conversation), which I admit I have not yet tried. 
  
  **Additional context**
  The issue and Slack conversation referenced above established a need for a custom adapter. I expect my issue could be solved by a customer adapter, too. I am happy to put in the learning and work to develop such an adapter, but I want to make sure I'm not missing something obvious before I head down that path.
  