Support JSON RPC methods `web3_clientVersion` and `eth_getStorageAt`

[abcoathup]

Support JSON RPC methods web3_clientVersion and eth_getStorageAt in loom-truffle-provider

OpenZeppelin SDK uses web3_clientVersion and eth_getStorageAt and is needed to support deploying OpenZeppelin SDK upgradeable contracts.

See ZWeb3.ts for web3 methods used:
https://github.com/OpenZeppelin/openzeppelin-sdk/blob/master/packages/lib/src/artifacts/ZWeb3.ts

For web3_clientVersion can addCustomMethod

const loomProvider = loomTruffleProvider.getProviderEngine()

loomProvider.addCustomMethod('web3_clientVersion', payload => {
  return 'Loom/v1.0.0/loom-js'
})