This adds `ci/azure-cleanup`, containing a script that talks to azure pipelines, removing agents older than 25 hours in a specific pool.
Machines are meant to be killed after 24 hours anyway, make sure they're properly unregistered from Azure Pipelines, too.
By doing this, we don't need to unregister nodes manually on shutdown.
Idea is to execute this every time a new agent is provisioned, it has cloned the repo. We intend to clone the repo and pre-warm the caches there anyhow.
WIP until the repo fetching and cache pre-warming is present, too.
cc @zimbatm
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