share: EDSStore.Put

[Wondertan]

Context

To write an entire EDS Put method is introduced on the EDSStore

Implementation Details

Internally it:

• Opens a file under Store.Path/DataRoot.Hash path
• Serializes the EDS into the file via share.WriteEDS
• Wraps it with DAGStore's FileMount
• Converts DataRoot's hash into the shard.Key
• Registers the Mount as a Shard on the DAGStore

NOTE: Registering on the DAGStore populates the top-level index with shares/proofs accessible from stored EDS, which is
out of the scope of the document.

// Put stores the given data square with DataRoot's hash as a key.
//
// The square is verified on the Exchange level, and Put only stores the square trusting it.
// The resulting file stores all the shares and NMT Merkle Proofs of the EDS.
// Additionally, the file gets indexed s.t. Store.Blockstore can access them. 
func (s *EDSStore) Put(context.Context, DataRoot, *rsmt2d.ExtendedDataSquare) error

Refs

Depends on #1105
Depends on #1107