Flint: Display flex 1D data

The flex device provides a 1D data without always the same amount of columns.

Talking to @sebastien.petitdemange the best way to publish the data is to use:

• 1 channel for the Y
• 1 channel for the X
• Metadata
  • Per device, in short term, fast way to display something
  • Metadata per channel to express the relationship between Y and X channels

To provide something working for any scans, the refactoring from #2692 (closed) is mandatory

To properly type the device, it would be good to refactor the scan_info #2687 (closed), #2705 (closed)

More or less related to #2683 (closed)