Various fixes & improvements
- feat(tracing): Dynamic Sampling Context / Baggage continuation (1485) by sl0thentr0py
The SDK now propagates the [W3C Baggage Header](https://www.w3.org/TR/baggage/) from
incoming transactions to outgoing requests.
It also extracts Sentry specific [sampling information](https://develop.sentry.dev/sdk/performance/dynamic-sampling-context/)
and adds it to the transaction headers to enable Dynamic Sampling in the product.