[Heartbeat] 'complete' Agent image (synthetics deps included)

[andrewvc]

Takes over from #22932

This issue covers the creation of a new elastic agent docker image that contains larger dependencies that would not be a good fit for the baseline elastic agent image. Specifically this covers the dependencies needed for synthetics which significantly increase the size of the agent image. While alternate approaches such as auto downloading synthetics images #26153 might make sense in the future this approach will be needed for those who would rather not download dependencies, some of which are quite sizable, on demand.

While the precipitating factor here is heartbeat we are naming this image more generically "off-line" for the reason that other things might fall in this category in the future.

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[kuisathaverat]

Probably, we can reuse some of the work made for the IronBank Docker images, these are the dependencies to install synthetics about 200MB on RPM packages https://repo1.dso.mil/dsop/elastic/beats/heartbeat/-/blob/development/hardening_manifest.yaml, we have scripts to grab the list of dependencies.

[andrewvc]

Closing, this is done in #27052