types/config, types/node: model default-auto-update from auto_update.enabled

Tailscale stamps tailcfg.NodeAttrDefaultAutoUpdate on every node's
CapMap with a JSON bool reflecting the tailnet-wide auto-update
default. Headscale grows an auto_update.enabled config option and
emits the cap accordingly from TailNode -- the cap leaves the
unmodelledTailnetStateCaps strip list and is compared in full by the
nodeAttrs compat suite.

testNodeAttrsSuccess drives cfg.AutoUpdate.Enabled from
tf.Input.Tailnet.Settings.DevicesAutoUpdatesOn so each capture's
expected emission matches the SaaS state it was taken under. Two
captures cover both branches:

  - nodeattrs-tailnet-devices-auto-updates-on  -> [true]
  - nodeattrs-tailnet-devices-auto-updates-off -> [false]

The Tailscale v2 TailnetSettings API does not expose the Send Files
toggle, so the compat suite cannot vary cfg.Taildrop.Enabled per
capture. TestTaildropDisabledWithholdsFileSharingCap covers the off
path directly in servertest.