share: ReadEDS

[Wondertan]

Context

As per #1099, we want to store EDSes as a whole. For this, the CAR format is chosen.

Implementation Details

To read an EDS from a stream/file, ReadEDS is introduced. Internally, it

• Imports EDS with an empty pre-allocated slice. NOTE: Size can be taken from DataRoot Row/Col len
• Wraps given io.Reader with BlockReader
• Reads out blocks one by one and fills up the EDS quadrant via EDS.SetCell
  • In total, there should be shares-in-quadrant amount of reads.
• Recomputes and validates via EDS.Repair

// ReadEDS reads an EDS quadrant(1/4) from an io.Reader CAR file.
// It expects strictly first EDS quadrant(top left).
// The returned EDS is guaranteed to be full and valid against the DataRoot, otherwise ReadEDS errors.
func ReadEDS(context.Context, io.Reader, DataRoot) (*rsmt2d.ExtendedDataSquare, error)