Turn credentials generation (#102)

* abstract peer channel

* remove wip code

* refactor NewServer with Peer updates channel

* feature: add TURN credentials manager

* hmac logic

* example test function

* test: add TimeBasedAuthSecretsManager_GenerateCredentials  test

* test: make tests for now with hardcoded secret

* test: add TimeBasedAuthSecretsManager_SetupRefresh test

* test: add TimeBasedAuthSecretsManager_SetupRefresh test

* test: add TimeBasedAuthSecretsManager_CancelRefresh test

* feature: extract TURNConfig to the management config

* feature: return hash based TURN credentials only on initial sync

* feature: make TURN time based secret credentials optional

Co-authored-by: mlsmaycon <mlsmaycon@gmail.com>

util/duration.go

@@ -0,0 +1,37 @@
+package util
+
+import (
+	"encoding/json"
+	"errors"
+	"time"
+)
+
+//Duration is used strictly for JSON requests/responses due to duration marshalling issues
+type Duration struct {
+	time.Duration
+}
+
+func (d Duration) MarshalJSON() ([]byte, error) {
+	return json.Marshal(d.String())
+}
+
+func (d *Duration) UnmarshalJSON(b []byte) error {
+	var v interface{}
+	if err := json.Unmarshal(b, &v); err != nil {
+		return err
+	}
+	switch value := v.(type) {
+	case float64:
+		d.Duration = time.Duration(value)
+		return nil
+	case string:
+		var err error
+		d.Duration, err = time.ParseDuration(value)
+		if err != nil {
+			return err
+		}
+		return nil
+	default:
+		return errors.New("invalid duration")
+	}
+}