caddy-frontend: Add Cache Lookup URL to the log

The cluc from https://docs.trafficserver.apache.org/en/7.1.x/admin-guide/logging/formatting.en.html#cache-details
allows to track the log entry per the originated request, which basically
makes debugging of ATS behaviour possible.

/reviewed-on !689
1 job for kvm-slaprunner-py3
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Master