fix: nrslog handler slog.Group

[luispe]

added processing for slog.Group when nested withing a log

Details

When a log.Group is added in newrelic it is replaced with a map key value that modifies the original values and empty values are stored.

e.g. of code:
https://github.com/newrelic/go-agent/tree/master/v3/integrations/logcontext-v2/nrslog/example

package main

import (
	"log/slog"
	"os"
	"time"

	"github.com/newrelic/go-agent/v3/integrations/logcontext-v2/nrslog"
	"github.com/newrelic/go-agent/v3/newrelic"
)

func main() {
	app, err := newrelic.NewApplication(
		newrelic.ConfigAppName("my-wesome-nrapp"),
		newrelic.ConfigFromEnvironment(),
		newrelic.ConfigAppLogEnabled(true),
	)
	if err != nil {
		panic(err)
	}

	app.WaitForConnection(time.Second * 5)
	log := slog.New(nrslog.TextHandler(app, os.Stdout, &slog.HandlerOptions{}))

	log.Info("I am a log message")

	log.Info("I am a log group inside a log message",
		slog.String("foo", "bar"),
		slog.Int("answer", 42),
		slog.Group("user",
			slog.String("name", "John Smith"),
			slog.String("email", "[email protected]"),
			slog.Int("age", 23),
		),
	)

	log.Info("All Done!")

	app.Shutdown(time.Second * 10)
}

Which is displayed in newrelic/logs

{
  "answer": 42,
  "entity.guid": "some_entity_guid",
  "entity.guids": "some_entity_guids",
  "entity.name": "my-wesome-nrapp",
  "foo": "bar",
  "hostname": "hostname_example",
  "level": "INFO",
  "message": "I am a log group inside a log message",
  "newrelic.logPattern": "nr.DID_NOT_MATCH",
  "newrelic.source": "logs.APM",
  "timestamp": 1735243287475,
  "user": "[{\"Key\":\"name\",\"Value\":{}},{\"Key\":\"email\",\"Value\":{}},{\"Key\":\"age\",\"Value\":{}}]"
}

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