Consider EDS helper Roots

[rootulp]

Context

https://github.com/celestiaorg/celestia-node/pull/2458/files#diff-c33387bdad1a81ed3928f505eadee22744b6fca98b897cacbcb918402843a4cbR246

Proposal

Consider adding a Roots() helper to EDS. Something like:

// Roots returns a byte slice with this eds's RowRoots and ColRoots
// concatenated.
func (eds *ExtendedDataSquare) Roots() (roots [][]byte, err error) {
	rowRoots, err := eds.RowRoots()
	if err != nil {
		return nil, err
	}
	colRoots, err := eds.ColRoots()
	if err != nil {
		return nil, err
	}

	roots = make([][]byte, 0, len(rowRoots)+len(colRoots))
	roots = append(roots, rowRoots...)
	roots = append(roots, colRoots...)
	return roots, nil
}

[bao1029p]

Hi guys, can we work on this ?

[rootulp]

Sure go for it 👍

[bao1029p]

@rootulp hey i created a PR can you give approval for the CI to run