feat(caddy): add Caddy support for Outline services

[sbruens]

This proof of concept lays the foundation for integrating Outline service support into Caddy through the implementation of two new Caddy modules:

1. outline Host Module: This module serves as the central hub for server-wide configuration settings related to the Outline service. It also handles the configuration of application-level metrics, ensuring we can monitor the service's performance effectively.

2. shadowsocks Layer4 App Guest Module: This module introduces a specialized handler to manage both stream-based and packet-based Shadowsocks connections. It operates in conjunction with the outline host module, utilizing its shared configuration settings.

---

Note:

While this PR establishes the core functionality, there are outstanding tasks related to metrics that might be addressed in separate PRs for better organization and focus:

• Metric Prefixing: Evaluate the necessity of aliasing or modifying the shadowsocks_ prefix, particularly for server-wide metrics like {PREFIX}_build_info, and {PREFIX}_ports, which aren't specifically tied to Shadowsocks.
• ip2info Support: Implement support for ip2info configuration within the outline module.
• Key and Port Metrics: Track {PREFIX}_keys and {PREFIX_ports} metrics in this Caddy implementation.

[fortuna]

Something seems wrong here. Is OutlineApp both an App and a Module?
You create an OutlineApp to then create another OutlineApp? That's a circular dependency and looks like a bug.

[fortuna]

This API is so broken... I think we should create something like:

type ModuleRegistration caddy.ModuleInfo

func (m ModuleRegistration) CaddyModule() caddy.ModuleInfo {
  return m
}

func init() {
  caddy.RegisterModule(ModuleRegistration{
    ID: "outline"
    New: func() caddy.Module { return new(OutlineApp) }
  })
}

[sbruens]

Something seems wrong here. Is OutlineApp both an App and a Module?

In Caddy apps are modules. So yes, OutlineApp is both an App and a Module.

You create an OutlineApp to then create another OutlineApp? That's a circular dependency and looks like a bug.

Yeah it's a bit of an odd API. I don't think we can get around it though; OutlineApp must implement caddy.Module.

[sbruens]

Discussed offline. I misunderstood the suggestion. Will split them out.

[fortuna]

Ugh, this API is so bad. Typical global registration badness, unfortunately... I had to deal with that a lot at Google.

I would still take the ModuleRegistration approach, and change OutlineApp to have a constructor that takes the replayCache as input. This is a more extensible approach and doesn't confuse the OutlineApp lifecycle (otherwise you have two OutlineApp objects!).

In order to do that, I think you will need to add a dummy CaddyModule() method to OutlineApp so the compiler doesn't complain, but that can just log error and return.

[sbruens]

Note we still need to provide a ModuleInfo with an accurate ID, so there is some duplication here now, but New() isn't used from it as far as I can tell. This is also why I'm not logging any errors here, since it is called and used in a few places.

[fortuna]

That's unfortunate. At least we don't have the two module objects co-existing anymore. Thanks for the changes.

[fortuna]

That's unfortunate. At least we don't have the two module objects co-existing anymore. Thanks for the changes.