Use FreeBSD autoscaler for "stress" tests

Use FreeBSD autoscaler for "stress" tests

The FreeBSD autoscaler has been configured to utilize the new "instance"
GitLab Runner executor to spawn "stress" test CI jobs on AWS EC2
dynamically. A shared GitLab Runner named "freebsd-instance-autoscaler"
has been set up in GitLab CI/CD to communicate with EC2, provisioning VM
instances on demand based on a FreeBSD 13 AMI image. This image is the
same as the one previously used for FreeBSD "stress" tests before the
implementation of autoscaling (specifically, the
"freebsd13-amd64-bind9stress.aws.lab.isc.org" GitLab Runner in CI/CD).

(cherry picked from commit 65a93b77e1804cd47e6f56137a107297eab94e12)