store/provide remote metadata for Transfers

[jdcasey]

It would be very useful if galley would store metadata related to the retrieval of remote artifacts, then provide a mechanism in the Transfer class to read/query it.

For http connections, this would take the form of filing away the response status code/message, along with all of the response headers. It could be very useful in detecting content types, or diagnosing upstream connection problems...or even helping advise the not-found cache on whether to cache a missing artifact or not.