Add support for WebSocket

[sbruens]

This tracks the main work towards supporting WebSocket in the Outline core server. It only covers the main changes, smaller cleanups are omitted. For the client-side work, see Jigsaw-Code/outline-apps#2272.

To Do

• refactor: move IP validation to the PacketListener #229

Done

We had to re-write major portions of our core server code to make it composable and add support for new protocols.

Update the config format to allow for more complex configurations beyond Shadowsocks

• refactor: remove use of port in probing metric #185
• refactor: move to a new "Service" config format #182
• refactor(server): use the new service config format outline-server#1628

Allow hot swapping of configs to avoid connection interruptions

• refactor: allow hot swapping the config #192
• refactor: decouple the TCP handler from the listener #193

Make logging composable

• refactor: add structured logging with slog #199
• refactor: pass in logger to service so caller can control logs #207

Make metric handling composable

• refactor: outlineMetrics implements prometheus.Collector #201
• refactor: decouple IPInfo from the service handlers #200

Add a webserver and Websocket handlers

• feat: add WebSocket support to the existing outline-ss-server #225
• refactor: use the SDK's WebSocket Upgrade() function #234
• feat: get client IP from the request headers #233

Create a Caddy module to integrate Outline services into Caddy

• refactor: split out UDP association handling from serving #221
• feat(caddy): add Caddy support for Outline services #198
• refactor: add structured logging with slog #199
• refactor: create re-usable Shadowsocks service #206
• feat(caddy): persist replay cache across config reloads #212
• feat(caddy): add a WebSockets handler that connects to Shadowsocks handler #216