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"
```