Add JSON schema for analysis file format 3.0.

This schema can be used to validate an analysis file using version 3.0 by using the following commands:

pip3 install jsonschema
unxz final_onionperf.analysis.json.xz
jsonschema -i final_onionperf.analysis.json onionperf-3.0.json

Implements tpo/metrics/onionperf#40003.