feat(linux): compute device ID from /etc/machine-id (#10805)

All of our Linux applications have a soft-dependency on systemd. That
is, in the default configuration, we expect systemd to be present on the
machine. The only exception here are the docker containers for Headless
Client and Gateway.

For the GUI client in particular, systemd is a hard-dependency in order
to control DNS on the system which we do via `systemd-resolved`. To
secure the communication between the GUI client and its tunnel process,
we automatically create a group called `firezone-client` to which the
user gets added. All members of the group are allowed to access the unix
socket which is used for IPC between the two processes. Membership in
this group is also a prerequisite for accessing any of the configuration
files.

On the first launch of the GUI client on a Linux system, this presents a
problem. For group membership changes to take the effect, the user needs
to reboot. We say that in the documentation but it is unclear whether
all users will read that thoroughly enough. To help the user, the GUI
client checks for membership of the current user in the group and alerts
the user via a dialog box if that isn't the case. This would all be fine
if it would actually work. Unfortunately, that check ends up being too
late in the process. If we aren't a member of the group, we cannot read
the device ID and bail early, thus never reaching the check and
terminating the process without any dialog box or user-visible error.

We could attempt to fix this by shuffling around some of the startup
init code. That is a sub-optimal solution however because it a) may get
broken again in the future and b) it means we have to delay
initialisation of telemetry until a much later point.

Given that this is only a problem on Linux, a better solution is to
simply not rely on the disk-based device ID at all. Instead, we can
integrate with systemd and deterministically derive a device ID from the
unique machine ID and a randomly chosen "app ID".

For backwards-compatibility reasons, the disk-based device ID is still
prioritised. For all new installs however, we will use the one based on
`/etc/machine-id`.

rust/headless-client/src/main.rs

@@ -246,7 +246,7 @@ fn try_main() -> Result<()> {
     // AKA "Device ID", not the Firezone slug
     let firezone_id = match cli.firezone_id.clone() {
         Some(id) => id,
-        None => device_id::get_or_create().context("Could not get `firezone_id` from CLI, could not read it from disk, could not generate it and save it to disk")?.id,
+        None => device_id::get_or_create_client().context("Could not get `firezone_id` from CLI, could not read it from disk, could not generate it and save it to disk")?.id,
     };
 
     let mut telemetry = if cli.is_telemetry_allowed() {