Enable log levels

[levaitamas]

This PR uses the logging levels of the slog package.

Note1: this change slightly alters the output (e.g., [DEBUG] -> DEBUG) and is half-assed.
Note2: if output breaking is not an issue, we can utilize the full power of structured logging in a PR later.

Meanwhile, the loglevel configuration is a matter of calling slog.SetLogLoggerLevel(loglevel) in the example as (the default level is Info):

package main

import (
	"log/slog"

	"github.com/netascode/go-sdwan"
)

func main() {
	slog.SetLogLoggerLevel(slog.LevelDebug)

	client, _ := sdwan.NewClient("1.1.1.1", "user", "pwd", true)

	res, _ := client.Get("/admin/resourcegroup")
	println(res.Get("0.id").String())
}