Nit UX Improvement: Add Provider Address in API Response

[ckartik]

It would be nice to check the commitmentSignature and do an ECRecover on the payload, and augment the output of the API to include the address associated with the provider.

e.g, currently if you POST a bid to the bidder api, you get back a payload:

{
      "txHash":"91a89B633194c0D86C539A1A5B14DCCacfD47094",
      "bidAmount":"2000",
      "blockNumber":"890200",
      "receivedBidDigest":"61634fad9081e1b23a00234a52a85386a91fc0fad3fb32d325891bdc4f8a52a9",
      "receivedBidSignature":"be99f499be5483665d569bd52fa8c4c9ed5c3bed2f94fb9949259bf640d8bd365b1d9a4b9ccb3ace86ffdcd1fdadc605b0f49b3adaf1c6a9ffea25585234664f1c",
      "commitmentDigest":"08a98d4c9d45f8431b46d99a23e6e8f82601ccf0773499d4e47bcd857cad92a6",
      "commitmentSignature":"cfc1e4e2ea9cc417027ed5846fa073c8e6031f70f91ffa9b92051b5b7738c0500d8fefb0fc189bce16004b3c0e1c4cfa41260968161adf8b55446ed5c40d1ac51b"
   }

We can augment the response to have "providerAddress": 0xProvider

{
      "providerAddress": 0xProvider
      "txHash":"91a89B633194c0D86C539A1A5B14DCCacfD47094",
      "bidAmount":"2000",
      ..."commitmentSignature":"cfc1e4e2ea9cc417027ed5846fa073c8e6031f70f91ffa9b92051b5b7738c0500d8fefb0fc189bce16004b3c0e1c4cfa41260968161adf8b55446ed5c40d1ac51b"
   }
}