Allow pushing Metrics to CollecTor from trusted endpoints

Switch from pull to push model for archiving OnionPerf data: Another aspect related to collecting data is that, right now, data collection works by periodically pulling new .tpf files from known OnionPerf instances. This has at least two problems: there's a delay between OnionPerfs producing new files and CollecTor pulling them, and adding new instances requires editing a config file on the CollecTor host. Maybe we can switch to a push model where CollecTor accepts measurements from any OnionPerf instance, and CollecTor clients like the Tor Metrics website decide which measurements to aggregate and visualize. Note that switching to a push model requires installing some basic authentication mechanisms like cryptographic identities and signatures, in order to prevent anyone from pushing wrong data, overwriting correct data, or even storing arbitrary data.