experimental/clashapi: include user in connection metadata

[yagjzx]

The InboundContext.User field is populated by vless / hysteria2 / anytls inbounds with the matched users[].name, but it is not currently included in the JSON output of /connections in clashapi. Without it, downstream consumers (Clash Meta-compatible dashboards, custom traffic accounting) cannot attribute traffic to a configured user.

This adds a single field to the marshalled metadata.

Behaviour

• For inbounds where a user has been matched, the field returns the configured users[].name (e.g. "user": "alice").
• For inbounds where no user matching applies (mixed, socks, http, etc.), the field serializes to the empty string. This is non-breaking for any consumer that ignores unknown fields.

Verification

Built locally with the project's standard tags. Spun up a vless inbound with two named users (alice, bob); a client connecting with the alice UUID correctly produced "metadata": { ..., "user": "alice" } in the /connections response. Connections through a non-user-bearing mixed inbound on the same instance correctly produced "user": "".

Why this is useful

Per-user traffic accounting is a common need for any multi-tenant proxy deployment. Today the only way to recover user attribution from clashapi is to give every user their own inbound.tag (which works via metadata.type) — but that explodes the listening-port footprint and requires per-tenant TLS material. Exposing the existing User field is a one-line, zero-cost alternative.