Feat: HTTP client supporting Brotli Content-Encoding

[chise0713]

I found that brotli is already an indirect dependency, so this PR adds a support for Content-Encoding: br.

Tested by this example:

package main

import (
        "net/http"

        "github.com/andybalholm/brotli"
)

func main() {
        http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
                data := `{"version":1,"rules":[{"domain_suffix":["alphabet.com"],"domain_keyword":["fitbit","google"]}]}`
                w.Header().Set("Content-Encoding", "br")
                w.Header().Set("Content-Type", "application/json")
                bw := brotli.NewWriter(w)
                defer bw.Close()
                bw.Write([]byte(data))
        })
        panic(http.ListenAndServe("127.0.0.1:1408", nil))
}

sing-box logs and config file:

➜ ./sing-box run -c test.json; cat test.json 
INFO[0000] network: updated default interface enp7s0, index 2
DEBUG[0000] router: updating rule-set test from URL: http://127.0.0.1:1408
INFO[0000] outbound/direct: outbound connection to 127.0.0.1:1408
INFO[0000] router: updated rule-set test
INFO[0000] sing-box started (0.00s)
^C{
  "route": {
    "rule_set": [
      {
        "type": "remote",
        "tag": "test",
        "format": "source",
        "url": "http://127.0.0.1:1408"
      }
    ]
  }
}

But it's only in rule_set_remote.go and router.go, and wrote by some random AI that's not me.

If this PR doesn't meet the standards, feel free to just close it.