Consider changing `host-containers@.service`'s `Type` to `idle`

We're adding serial console access support to Bottlerocket's admin container in https://github.com/bottlerocket-os/bottlerocket-admin-container/pull/59. We might want to avoid interleaving the admin container's output with other systemd unit status output in the console.

By changing `host-containers@.service`'s process start-up type to `idle`, the admin container's login prompt will show up towards the end and not get buried under other console output from other units since it'll wait until all active systemd jobs are dispatched. This would be consistent with how other distros handle serial console logins with `getty@.service`s. To quote https://wiki.archlinux.org/title/getty:
> The option Type=idle found in the default getty@.service will delay the service startup until all jobs (state change requests to units) are completed in order to avoid polluting the login prompt with boot-up messages.

Another behavior change here would be that host-containers would run later in the boot sequence after all other host services start. This is OK since it makes host-containers start-up timing/ordering more consistent in the overall boot sequence.

See more about the different process start-up types here: https://www.freedesktop.org/software/systemd/man/systemd.service.html#Type=

Locate the host-containers@.service unit definition and read how its current Type and startup ordering are configured. Change the service type to idle, then verify that host containers still start successfully and that the login prompt is delayed until other active systemd jobs have been dispatched.

Written by the indexing model from the issue text.