server: optional log of served device config

[eriknordmark]

Summary

Adam logs every inbound API request (requested /api/v2/edgedevice/id/<UUID>/config).
For investigations where you also need to see what was returned —
which baseos, which content tree, how many apps/networks etc. — the
request log alone isn't enough.

This PR adds a one-line response summary, gated by an env var so the
default log volume is unchanged.

Behaviour

With ADAM_LOG_DEVICE_CONFIG=1 set, every served device config emits
on stderr:

config served: uuid=<UUID> version=<N> baseos.version=<V> \
    baseos.activate=<bool> baseos.ct=<UUID> baseosconfig_list=<n> \
    contentInfo=<n> apps=<n> networks=<n> volumes=<n> datastores=<n>

Off by default → no log-volume cost for normal operators.

The log fires before the 304 Not Modified short-circuit so it
surfaces the response shape on every poll, even those where EVE
already has the current config. That makes it useful for "is EVE
even polling for /config right now?" questions, not just "what's in
the response?".

How to enable

# bare adam
ADAM_LOG_DEVICE_CONFIG=1 adam server

# adam container
docker run -e ADAM_LOG_DEVICE_CONFIG=1 lfedge/adam server

# eden-managed adam
docker run -e ADAM_LOG_DEVICE_CONFIG=1 ... lfedge/adam:<tag> ...

docker logs eden_adam (or wherever stderr lands) then carries one
config served: line per /config poll.

Field choice

The fields cover the categories the response body decomposes into —
baseos block, baseos list (legacy), contentInfo, apps, networks,
volumes, datastores. Each is summarised by either a key fact
(baseos: version + activate + content-tree UUID) or a count (lists).
That keeps the line a single grep target while still answering the
common debugging questions:

• "Did adam send a baseos this poll?" — baseos.version
• "Is it asking EVE to install it?" — baseos.activate
• "Why does my contentInfo count differ across polls?" — contentInfo=N
• "How long since the controller's last edit?" — version=N

Test

make build
ADAM_LOG_DEVICE_CONFIG=1 ./bin/adam-linux-amd64 server ...   # then poll /config
# observe one "config served: uuid=..." line per poll on stderr
unset ADAM_LOG_DEVICE_CONFIG
./bin/adam-linux-amd64 server ...                            # poll /config
# observe no extra log lines (only the existing request-line)

Unit tests (go test ./...) pass.