Add parsing support for OnionPerf analysis files

Back in the days, Torperf wrote its results to .tpf files using one line per measurement with space-separated key=value pairs. OnionPerf continued using this format in addition to writing a more sophisticated JSON format. It's time to teach metrics-lib how to read that JSON format, so that we can abandon the .tpf format. That is going to simplify upcoming changes to OnionPerf where we then only have to write a single output format.

The first step is to add parsing support for OnionPerf analysis files to metrics-lib. I worked on a patch over the past weeks that parses JSON files and internally converts them to Torperf results. The idea here is to keep changes to applications like metrics-web as small as possible. They can simply read different descriptor files and obtain the same output as before. We can later extract more parts from OnionPerf analysis files and provide them in a new interface, but we don't have to do that right now.