Adds a JsonObjectSpanWExporter and an associated contextmanager to support testing of span creation
What's Changed
* Add JsonObjectSpanExporter and context manager by keithralphs in https://github.com/DiamondLightSource/observability-utils/pull/16
**Full Changelog**: https://github.com/DiamondLightSource/observability-utils/compare/0.1.2...0.1.3