Feature: High-level boundary management interface

[beryll1um]

I believe that any server-side management logic requires high-level boundary management interfaces to implement it. Manually restarting the service with a new configuration is not the best choice for a high-loaded environment where many users and protocols are handled.

Such interfaces are already implemented in v2fly/v2ray-core, but I find their implementation generally messy, so I prefer to switch to sing-box. Unfortunately, such functionality is not implemented there yet, as far as I understand...
I decided to implement this myself, so check out my contribution. I'm still working on it, but overall it's done and working fine as far as I can verify.

I have some suspicions that this might break something, because some experimental tools like the Clash API can get a pointer to outgoing connections to work with them separately, and during work time these outgoing connections can be removed from the router, but so far I haven't found any problems with this (doesn't means there is no of them).

Please check my pull request: #2177

[beryll1um]

How it looks like at this time:

package main

import (
	"context"
	"net/netip"
	"time"

	"github.com/sagernet/sing-box"
	"github.com/sagernet/sing-box/option"

	C "github.com/sagernet/sing-box/constant"
)

func main() {
	boxInstance, err := box.New(box.Options{
		Context: context.TODO(),
		Options: option.Options{
			Inbounds: []option.Inbound{
				{
					Tag:  "shadowsocks",
					Type: C.TypeShadowsocks,
					ShadowsocksOptions: option.ShadowsocksInboundOptions{
						ListenOptions: option.ListenOptions{
							Listen:     option.NewListenAddress(netip.IPv4Unspecified()),
							ListenPort: 1024,
						},
						Method:   "chacha20-ietf-poly1305",
						Password: "1234567890",
					},
				},
			},
			Outbounds: []option.Outbound{
				{
					Tag:  "direct",
					Type: C.TypeDirect,
				},
			},
		},
	})
	if err != nil {
		panic(err)
	}
	if err := boxInstance.Start(); err != nil {
		panic(err)
	}
	time.Sleep(10 * time.Second)
	if err := boxInstance.RemoveOutbound("direct"); err != nil {
		panic(err)
	}
	time.Sleep(10 * time.Second)
	if err := boxInstance.RemoveInbound("shadowsocks"); err != nil {
		panic(err)
	}
	time.Sleep(10 * time.Second)
	if err := boxInstance.Close(); err != nil {
		panic(err)
	}
}