Add a job results API for querying LAVA jobs

This provides a byte stream of the junit test results for a completed LAVA job, using the standard REST junit endpoint that LAVA provides in v0.2.

The reason to provide a raw bytestream, rather than parsed Junit data, is the difficulty of choosing any particular junit crate for rust at the present: none of them are comprehensive, and it really depends what you want to do with the data which you should pick. It seems safer for now to avoid having any policy here.