Propagate DSC through sentry-opentelemetry

[szokeasaurusrex]

Update sentry-opentelemetry::SentryPropagator so OpenTelemetry users get the same Sentry DSC propagation behavior as the core transaction path. The trace propagation spec 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, #1016.

[linear-code]

RUST-237