splunk_logger: move environment hook to splunk_logger pt2

for reusability also in image-builder

vendor/github.com/osbuild/osbuild-composer/pkg/splunk_logger/environment_hook.go

@@ -0,0 +1,26 @@
+package logger
+
+import (
+	"github.com/sirupsen/logrus"
+)
+
+type EnvironmentHook struct {
+	Channel string
+}
+
+func (h *EnvironmentHook) Levels() []logrus.Level {
+	return []logrus.Level{
+		logrus.DebugLevel,
+		logrus.InfoLevel,
+		logrus.WarnLevel,
+		logrus.ErrorLevel,
+		logrus.FatalLevel,
+		logrus.PanicLevel,
+	}
+}
+
+func (h *EnvironmentHook) Fire(e *logrus.Entry) error {
+	e.Data["channel"] = h.Channel
+
+	return nil
+}