- Allow for a "type" of backend when we create the `ServiceXDataset`
- Allow for multiple servicex backends to be specified in a .servicex file
This breaks the current user interface:
- You have to specify a `type` of backend when you create the `ServiceXDataset`
- If you use a `.servicex` file, you'll need to change the format so you can have multiple servicex endpoints in there.
While this release was built, and it does contain changes, we decided to back out the changes that broke the old UW - so we can go back to the 2.1 series.