It's useful to have a way to set the service name for all spans at once,
rather than having to set it every time.

This commit adds a with_service_name method to PipelineBuilder
and Exporter to allow users to set the service name in one place,
following the behaviour of the official DataDog and Zipkin exporters.

It also sets a default service_name if the user does not configure one.

Rather than relying on a property on PipelineBuilder, instead set the
service.name in the sdk::Config by merging sdk::Resouce properties. This
is also a change to the behavior of with_config.

Co-authored-by: isobelhooper <isobel.hooper@cambridgequantum.com>

Set service.name through sdk::Resource