Publish docker image to be used in tempo examples

[stoewer]

What this PR does:

This PR contains changes required to publish a xk6-client-tracing docker image that runs k6 to generate traces and send them to a OTLP or Jaeger endpoints:

• Make endpoints in examples configurable
• Add example script to docker image
• GitHub action to build and publish the image when a new release is created

The published docker image can also be used as a replacement for the synthetic-load-generator in Tempos' docker-compose examples. To run an example with xk6-client-tracing replace the load generator service in the respective docker-compose.yaml with the following snippet:

  k6-client-tracing:
    image: ghcr.io/grafana/xk6-client-tracing:latest
    environment:
      - ENDPOINT=tempo:4317
    restart: always
    depends_on:
      - tempo