Synchronize `PropagationContext` across hybrid layers

[stefanosiano]

Meta issue: getsentry/team-mobile#189

Currently, WebViews, RN and native layers keep their own traceId. We could enhance the debugging experience by sharing trace across the hybrid applications layers.

Examples

• RN opens WebView and an error (Browser SDK in WebView captured an error) happens on the web due to RN not supplying needed data (error was capture by RN). These are not sharing the same trace as the SDK operate independently.
• Native SDK captures error, due to which data are missing in RN layer, and thus RN captures an error. Currently these errors are no covered by the same trace id.