Enhance writing Gate config for external programs (fixes #325, closes #327)

[robinbraemer]

example program that can now rewrite the gate config seamlessly:

package main

import (
    liteconfig "go.minekube.com/gate/pkg/edition/java/lite/config"
    "go.minekube.com/gate/pkg/gate/config"
    "go.minekube.com/gate/pkg/util/configutil"
    "gopkg.in/yaml.v3"
    "os"
)

func main() {
    cfg := config.Config{}
    f, err := os.ReadFile("config.yml")
    if err != nil {
        panic(err)
    }
    err = yaml.Unmarshal(f, &cfg)
    if err != nil {
        panic(err)
    }
    cfg.Editions = config.Editions{} // empty it
    cfg.Config.Lite.Routes = append(cfg.Config.Lite.Routes, liteconfig.Route{
        Host:    configutil.SingleOrMulti[string]{"localhost"},
        Backend: []string{"localhost"},
    })
    out, err := yaml.Marshal(cfg)
    if err != nil {
        panic(err)
    }
    err = os.WriteFile("config.yml", out, 0644)
    if err != nil {
        panic(err)
    }
}

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