Add parent header hash and block number in getWork api

[atif-konasl]

Overview

Currently, getWork api does not take any parameters when it is called by validator client. For resolving multiple pandora block for single slot problem, getWork api needs to know about previous parent hash and block number from vanguard side and validator will provide this information to pandora client. When getWork gets these 2 value, then it will compare with its canonical chain's head's hash and block number. If they are matched then pandora will create new block.