share: implement getShareByRange

[vgonkivs]

Blob module should be extended with:

// GetSharesByRange returns the Shares in the range of [start; end) and their respective proofs, 
// where start and end are the positions of the Shares inside the data square
GetSharesByRange(start, end int) ([]Shares, []Proof, error)

[Wondertan]

I would keep it in ShareGetter, let's not mix shares in blobs

[vgonkivs]

Agree