Observability: Configurable service name for check or check group

When creating the root span, checkly uses the service name checkly to make it very obvious that it’s the one we originate that comes through with the Check requests.

It’d be nice to make this service name configurable to identify trends across applications or to have them grouped correctly in 3rd party services such as Honeycomb, which uses the service name to select the dataset spans get aggregated to.