RFE: common interface to expose host's os-release to container-like runtimes

The following was the start of a discussion regarding the possibility of establishing a common interface for exposing the host's os-release via ~container runtimes:

> We find ourselves in a situation where application running
> containerized (specifically as portable services, although this issue
> is general) need to either find information about the host, or alter
> their behaviour depending on the host's flavour.
> At the same time, applications also need to find information about the
> guest's OS, so simply bind mounting /etc/os-release from the host is
> not enough. We need both.

Reference: https://lists.freedesktop.org/archives/systemd-devel/2020-April/044378.html

The desired goal is to establish a common interface to provide this information, so that applications can rely on it without having to infer what runtime manager they are spawned by.

There are many ways to implement this, the discussion for the nspawn/portabled cases gravitated toward using prefixed environment variables - the advantage being that it's the most convenient way for applications, since they can trivially have both sets of information at the same time (eg: source /etc/os-release and then VERSION_ID is the runtime's version, service_host_VERSION_ID is the host's version).

RFC patches for nspawn/portabled/flatpak:

https://github.com/systemd/systemd/pull/15891
https://github.com/flatpak/flatpak/pull/3640

Thoughts?