Method for client to determine which payload formats are supported

[achimr]

-- It seems there is no way for the caller to know what kind of payload the provider can process. While it can likely be handled through error reporting, it'd be better to be able to query the capabilities of the provide before sending data. (reported by Yves Savourel)

HTTP already has content negotiation but it does not apply, it defines the flexible delivery of different content formats from server to client: http://www.w3.org/Protocols/rfc2616/rfc2616-sec12.html; we need another function that queries the provider for acceptable translation request formats