feat(baggage): add support for w3c baggage

[rachelyangdog]

What does this PR do?

Adds W3C Baggage propagation support (baggage header) to the composite propagator. When baggage is included in the propagation style configuration, the global TextMapPropagator will read and write the baggage HTTP header using opentelemetry_sdk::propagation::BaggagePropagator.

Changes:

• Adds TracePropagationStyle::Baggage variant, recognized case-insensitively from DD_TRACE_PROPAGATION_STYLE, DD_TRACE_PROPAGATION_STYLE_EXTRACT, and DD_TRACE_PROPAGATION_STYLE_INJECT

• Adds baggage to the default propagation styles alongside datadog and tracecontext

• Wires inject/extract separately at the DatadogPropagator layer, respecting the extract-vs-inject style override independently

• Adds propagation::baggage module exposing the header key so fields() includes "baggage" when the style is configured

• system-tests PR

Motivation

What inspired you to submit this pull request?

Additional Notes

• No new dependencies: opentelemetry_sdk::propagation::BaggagePropagator was already a transitive dependency
• Baggage propagation is orthogonal to trace context: it operates on OTel Context directly rather than through the internal Propagator trait (which is trace-ID-centric). The Baggage variant's extract/inject arms in that trait are intentional no-ops; the actual work happens one layer up in DatadogPropagator.
• DD_TRACE_PROPAGATION_STYLE_EXTRACT and DD_TRACE_PROPAGATION_STYLE_INJECT can be used to enable baggage on only one direction independently of the other

[bantonsson]

Looks good overall

[bantonsson]

What happens when we only have baggage as an injector or extractor?