Backport to have VSL add user-agent header

BCornwallrequested to merge
bullseye-wikimedia-vtc-add-user-agent-header-by-default into bullseye-wikimedia

This patch was backported from 7.4 so that user-agent throttling doesn't occur when running tests without having to add a "User-Agent" header to each and every txreq.

This was tested by locally installing this pipeline's varnish .debs in the docker_run.sh file. All tests pass except a few from 31-blocked-nets.vtc that assume that no header is set. That's fixed by passing the new -nouseragent flag (https://gerrit.wikimedia.org/r/c/operations/puppet/+/1180233)

Bug: T400119

Edited by BCornwall

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