scripts: runners: rework RunnerCaps implementation

This provides miscellaneous quality of life improvements:

- We couldn't use dataclasses when this class was originally written.
  We can now, so move to dataclass to avoid having to write
  __repr__().

- Add missing validation for the advertised commands

- Add missing documentation for the 'file' capability

Signed-off-by: Martí Bolívar <mbolivar@amperecomputing.com>