Add the ability to set resource attributes

[jordanbertasso]

I'd like to be able to set the deployment.environment resource attribute on my spans.

It would be nice if we could set any arbitrary resource attributes. At the moment we can only set service name as seen here: https://github.com/grafana/xk6-client-tracing/blob/e340ce862e2f8faa07cf3333864e3ae2d2fd183e/pkg/tracegen/templated.go#L233

Thanks!