Send workshop machine ID with workshopctl requests

[tlm]

Description

Forward the LXD-provided workshop machine ID from workshopctl to workshopd in the workshop-machine-id HTTP header.

This enables follow-up secrets work: workshopctl and workshopd need a shared workshop identity to communicate and validate the workshop from which a secret request originates. The daemon records the received ID in debug request logs to support diagnosis.

This PR intentionally only transports the identifier. It does not validate the ID or authorise secret access; those checks will be added in follow-up secrets PRs.

Self-review quick check

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Docs

• I confirm the PR has no implications for documentation.

[jonathan-conder]

just FYI from machine-id(5):

This ID uniquely identifies the host. It should be considered "confidential", and must not be exposed in untrusted environments, in particular on the network. If a stable unique identifier that is tied to the machine is needed for some application, the machine ID or any part of it must not be used directly. Instead the machine ID should be hashed with a cryptographic, keyed hash function, using a fixed, application-specific key. That way the ID will be properly unique, and derived in a constant way from the machine ID but there will be no way to retrieve the original machine ID from the application-specific one. The sd_id128_get_machine_app_specific(3) API provides an implementation of such an algorithm.

I don't think the socket necessarily counts as the "network," but it seems like a good idea regardless.