Fix credentials not being returned along with the request ID (#78)

Client expects credentials to be returned as a promise, but it can't
match the response to the request due to response ID not being set:

```typescript
if (packet.rid !== undefined) { // this is always false
        const request = this.requests.get(packet.rid)
        if (request != null) {
          this.requests.delete(packet.rid)
          if (packet.type === 'error') {
            request.reject(new SignalingError('server-error', packet.message))
          } else {
            request.resolve(packet)
          }
        }
      }
```

This causes TURN to be completely broken since no credentials are
available.

Tested locally by hardcoding some creds:

```go
func (c *CredentialsClient) GetCredentials(ctx context.Context) (*Credentials, error) {
	return &Credentials{
		URL:        "someURL",
		Username:   "bsusername",
		Credential: "bspassword",
		Lifetime:   123,
	}, nil
}
```

Happy to make adjustments as I have no experience in Go :)

internal/signaling/handler.go

@@ -104,17 +104,20 @@ func Handler(ctx context.Context, store stores.Store, cloudflare *cloudflare.Cre
 				util.ErrorAndDisconnect(ctx, conn, err)
 			}
 
-			typeOnly := struct{ Type string }{}
-			if err := json.Unmarshal(raw, &typeOnly); err != nil {
+			base := struct {
+				Type      string `json:"type"`
+				RequestID string `json:"rid"`
+			}{}
+			if err := json.Unmarshal(raw, &base); err != nil {
 				util.ErrorAndDisconnect(ctx, conn, err)
 			}
 
 			if peer.closedPacketReceived {
-				logger.Warn("received packet after close", zap.String("peer", peer.ID), zap.String("type", typeOnly.Type))
+				logger.Warn("received packet after close", zap.String("peer", peer.ID), zap.String("type", base.Type))
 				continue
 			}
 
-			switch typeOnly.Type {
+			switch base.Type {
 			case "credentials":
 				credentials, err := cloudflare.GetCredentials(ctx)
 				if err != nil {
@@ -123,6 +126,7 @@ func Handler(ctx context.Context, store stores.Store, cloudflare *cloudflare.Cre
 					packet := CredentialsPacket{
 						Type:        "credentials",
 						Credentials: *credentials,
+						RequestID:   base.RequestID,
 					}
 					if err := peer.Send(ctx, packet); err != nil {
 						util.ErrorAndDisconnect(ctx, conn, err)
@@ -140,7 +144,7 @@ func Handler(ctx context.Context, store stores.Store, cloudflare *cloudflare.Cre
 				// ignore, ping/pong is just for the tcp keepalive.
 
 			default:
-				if err := peer.HandlePacket(ctx, typeOnly.Type, raw); err != nil {
+				if err := peer.HandlePacket(ctx, base.Type, raw); err != nil {
 					util.ErrorAndDisconnect(ctx, conn, err)
 				}
 			}

internal/signaling/types.go

@@ -96,7 +96,8 @@ type ForwardablePacket struct {
 
 type CredentialsPacket struct {
 	cloudflare.Credentials
-	Type string `json:"type"`
+	Type      string `json:"type"`
+	RequestID string `json:"rid,omitempty"`
 }
 
 type EventPacket struct {