[feature] add support for sending headers to tracing system

Add support for sending additional HTTP or gRPC headers which
can be used for authentication or other additional information
for the tracing system without having to set up a local instance
of the OpenTelemetry Collector to add these headers.

Example with Dash0:

```yaml
tracing-enabled: false
tracing-transport: "grpc"
tracing-endpoint: "ingress.eu-west-1.aws.dash0.com:4317"
tracing-headers:
  "Authorization": "Bearer DASH0_AUTH_TOKEN"
  "Dash0-Dataset": "gotosocial"
```

Example with Honeycomb:

```yaml
tracing-enabled: false
tracing-transport: "grpc"
tracing-endpoint: "api.honeycomb.io:443"
tracing-headers:
  "x-honeycomb-team": "YOUR_API_KEY"
  "x-honeycomb-dataset": "YOUR_DATASET"
```

internal/tracing/tracing.go

@@ -60,6 +60,7 @@ func Initialize() error {
 	case "grpc":
 		opts := []otlptracegrpc.Option{
 			otlptracegrpc.WithEndpoint(config.GetTracingEndpoint()),
+			otlptracegrpc.WithHeaders(config.NewState().GetTracingHeaders()),
 		}
 		if insecure {
 			opts = append(opts, otlptracegrpc.WithInsecure())