Allow client to post {verifiablePresentationRequest: {...}} when initiating exchange

[dlongley]

Enable clients to signal that they would like to see what is being offered by the exchange in their opening POST. So, instead of just posting {} (empty object) to start an exchange, allow a client to POST {verifiablePresentationRequest: {...}} to receive information about the exchange (in the form of a VPR) or to request a VP from the server before they are willing to engage.