Allow to use the spreadsheet iterator as a migrate_plus data parser

Problem/Motivation

For some use cases it could be useful to be able to migrate data directly from a URL (XLS file stored on an external server for example).
The migrate_plus module easily allows to fetch data from URLs instead of local files and provide a data_parser plugin mecanism to read this data.

Proposed resolution

Create a migrate_plus data_parser plugin to be able to read files from a URL instead of a file.