Reapply "Custom headers"

[mikhailm-coder]

Reverts 744876e, restoring 3cfbf0f.

custom_header has been running in production this whole time — openframe-client pins
rev = 3cfbf0f precisely because it needs it, and uses it in four places:

logging/nats_streaming.rs:52-54	x-tenant-domain, x-initial-key, x-machine-id
services/nats_connection_manager.rs:132	X-MACHINE-ID

x-initial-key is that connection's only credential, so the feature is load-bearing, not
experimental. With it absent from main, the client cannot build against main at all and is
stuck on an orphaned commit.

This restores it verbatim so the client can rejoin main's lineage. No behaviour change for
anything on main today, which has no custom_header callers.

Stacked on top of this: #106 — makes the headers resolvable per connect attempt so a
rotating bearer token can live in one.

🤖 Generated with Claude Code

https://claude.ai/code/session_0187LY8GpfSvFDqYN1cDAi9u