Propagate DSC through sentry-opentelemetry

Update `sentry-opentelemetry::SentryPropagator` so OpenTelemetry users get the same Sentry DSC propagation behavior as the core transaction path. The [trace propagation spec](https://develop.sentry.dev/sdk/foundations/trace-propagation/) requires SDK propagation mechanisms to carry both `sentry-trace` and `baggage`, while the current OTel propagator handles only `sentry-trace`.

Extract `baggage` alongside `sentry-trace`, inject Sentry DSC as `baggage`, include `baggage` in `SentryPropagator::fields()`, and pass extracted DSC to the transaction context created by the OTel processor. Keep this issue propagation-only; Sentry DSC should not control OTel sampler behavior. Related: [#741](https://github.com/getsentry/sentry-rust/issues/741), [#1016](https://github.com/getsentry/sentry-rust/issues/1016).