Introduce ApplicationConfig.ExposeNodeHeader plus the matching
--expose-node-header / LOCALAI_EXPOSE_NODE_HEADER CLI flag. Default off
because the node ID identifies internal cluster topology and should not
be advertised on a public endpoint; operators opt in for debugging,
observability and load-balancer attribution. The wiring lands first as a
no-op so the follow-up handler patch has a config field to read.
Assisted-by: Claude:claude-opus-4-7[1m]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>