Add DSC baggage parsing and serialization

Add W3C `baggage` conversion for `DynamicSamplingContext` so sentry-rust can read and write Sentry DSC values in the format required by the [Dynamic Sampling Context spec](https://develop.sentry.dev/sdk/foundations/trace-propagation/dynamic-sampling-context/#baggage-header). The implementation should parse `sentry-*` baggage entries into known DSC fields, serialize populated DSC fields as `sentry-=`, handle percent-decoding and percent-encoding, and expose the getters needed by `sentry-core`.

Cover supported DSC fields, empty baggage, mixed vendor baggage, malformed values, percent-encoding, and the chosen behavior for unknown future `sentry-*` keys. Related: [#741](https://github.com/getsentry/sentry-rust/issues/741), [#1016](https://github.com/getsentry/sentry-rust/issues/1016).