created LivepeerAccount entity

[kautukkundan]

Created LivepeerAccount entity abstraction that contains Delegator and Delegate details for any address that is bonded in the protocol.

closes #16

can be tested here for arbitrum-rinkeby
with the following query

{
  livepeerAccounts {
    id,
    delegator {
      id,
      startRound,
      bondedAmount,
    }
    delegate {
      id,
      activationRound,
      totalStake
    }
  }
}

[adamsoffer]

Looks great! 🛳

[0xcadams]

This should be merged into the main branch, @kautukkundan @adamsoffer do we still need this?