Use GCP code coverage in backend

[La0]

This PR replace the /v2/path endpoint to use GCP covdir data + a redis cache.

Organisational data is stored in redis:

• a list of changesets with their pushes (eg: changeset:deadbeef :arrow_right: 12345)
• a sorted set of the GCP reports found, sorted by their push id (eg: reports :arrow_right: {12345: "deadbeef", ...}

The coverage data is downloaded from GCP, extracted and stored in a local file path. :warning: No coverage data is available in Redis