(#22375) File bucket serialization

Previously, the agent was PSON encoding files when backing them up via REST. This is not reliable when transferring binary content, and is very slow, since it applies a regexp the entire file content.

Also, the master was echoing the file back to the agent in the HTTP response body, which is completely unnecessary. The master would YAML encode the file on the way back, unless you're using a newer master that respects the Accept content type specified by the agent (#21427).