Add DSC baggage parsing and serialization

[szokeasaurusrex]

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. The implementation should parse sentry-* baggage entries into known DSC fields, serialize populated DSC fields as sentry-<name>=<value>, 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, #1016.

[linear-code]

RUST-234