Basic IMDS support (#40980)

This adds a new cloud IMDS client, that can cover AWS, Azure, GCP,
Hetzner imds to varying degrees. Each cloud has this and it's very basic
functionality, hence I think it makes sense to add this to systemd.

Since the clouds are all different this tries hard to do the abstract
common logic in code, but encode the endpoint details, and well-known
keys in hwdb, attached to the DMI id device.

Why all this?

* Efficiency: we can schedule this in the initrd, at the earliest points
possible, without unnecessary delays
* Robustness: imds is typically slow and/or heavily rate-limited:
systemd-imdsd as single entrypoint can deal with that, and provide a
reliable, cached interface
* Security: the idea is that systemd-imdsd is the only service behing
able to access the IMDS HTTP, and the host carries a blackhole route for
it otherwise. That way sensitive info can be kept away from clients, and
requires polkit auth for access
* Simplicity: extraction of systemd's system credentials from IMDS
userdata happens with systemd's own infra, and for many usecases that
should already be enough.
* Measurements: before accepting the IDMS userdata, it can be measured
into a PCR, as any other configuration input for the system

Fixes: #40763