Change bidData in Auction::Respone to Json object instead of plain Strin...

Currently, exchange connector can only get the creative information from agentConfig object, which needs to be shared across different process, e.g: Router/PAL/Agent. It's not good to support application which has big number of creatives, which will make the configure object pretty huge. And, basically the idea is to give the flexibilty of agent to pass customized information to exchange connector on the fly.

Thanks Remi help me to get through the whole logic.
Will do and update the pull request.