As a follow-up of #1141 , we should add baggage header propagation as outline in the DSC docs for both incoming and outgoing HTTP requests.

For this, we would use the existing sentry_transaction_iter_headers for adding dsc to outgoing requests, and provide a function similar to (or as a part of) sentry_transaction_context_update_from_header for incoming requests (handling dsc in the baggage much like we already handle sentry-trace)

For now this is backlogged, and will be bumped in priority if there's some user demand for it.