An OpenTelemetry Protocol span exporter

Changes for 0.014 - 2023-11-23T19:32:20+00:00

• Bump Object::Pad dependency to 0.74 to ensure support for field initialisers
• Make all shutdown and force_flush methods async. This adds a new dependency on Future::AsyncAwait
• Fix issues with JSON encoding of timestamps, span and trace IDs, and string values