#990 Allow swamp clients to send custom headers when connecting to swamp serve

Problem

I want to run swamp serve behind a reverse proxy or tunnel. Some reverse proxies/tunnels require clients to include arbitrary HTTP headers before they will forward traffic to the backend. Today there does not appear to be a way for swamp client-side commands to attach those required headers when connecting to a swamp serve endpoint.

Desired behavior

Add a supported client-side configuration/flag/env-var mechanism for swamp clients to send custom HTTP headers when connecting to swamp serve.

Example use cases:

• Reverse proxy/tunnel authentication headers such as Tunnel-Token: ... or provider-specific access headers. (I need to send multiple headers!)
• Local/private deployments where access to the served swamp API is gated by a proxy that expects one or more static headers.

Possible UX

Any of these would work; exact design is up to the project:

• CLI flags, for example --header "Name: value", repeatable.
• Client config, for example a per-server list of headers.
• Environment variables for automation/CI.

Headers may contain secrets, so the implementation should avoid logging values and should document safe storage/retrieval guidance.

Acceptance criteria

• A user can configure one or more arbitrary HTTP headers on the client side.
• Those headers are sent on requests to a configured swamp serve endpoint.
• Header values are not exposed in normal logs/error output.
• Documentation explains how to use this behind a reverse proxy/tunnel.

Context

This is specifically for clients connecting to swamp serve through a reverse proxy/tunnel that enforces required request headers.