sysutils/py-google-compute-engine: revive port

This reverts commit 5a7ff5102154b08055c37868810192a9dca8044b and uses
www/py-boto3 instead of the removed devel/py-boto, and releng-gce@
assumes maintainership.

This is meant to be a temporary measure as we are exploring alternatives
for GCE support.

MFH: 2025Q2
(cherry picked from commit 592b450ab9275d2b5423132059ca9e2c231746d1)