feat(propagation): decouple propagation module from config and make it public

[duncanista]

What does this PR do?

Decouples the propagation module from the concrete Config struct by introducing a PropagationConfig trait. This makes DatadogCompositePropagator generic over any configuration type that implements the trait, and exports the propagation module unconditionally (previously
gated behind test-utils).

Changes:

• Add PropagationConfig trait with the 5 methods needed by propagators
• Make DatadogCompositePropagator<C: PropagationConfig> generic for zero-cost monomorphization
• Implement PropagationConfig for Config to maintain full backward compatibility
• Make propagation module unconditionally pub (was pub only under test-utils, pub(crate) otherwise)
• Re-export TracePropagationStyle from the propagation module

Motivation

External consumers like the Lambda extension (bottlecap) have their own propagation code that duplicates what lives here. By decoupling propagation from the full Config struct (which carries 70+ fields and heavy dependencies), external crates can depend on
datadog-opentelemetry for propagation by implementing the lightweight PropagationConfig trait on their own config type — without pulling in the entire tracer configuration.

Additional Notes

• All 285 existing propagation tests pass without modification (beyond adding the now-needed Config import in test modules).
• The Propagator trait remains pub(crate) — only PropagationConfig, DatadogCompositePropagator, and the context/carrier types are public.
• Uses generics (impl PropagationConfig) rather than dyn PropagationConfig since propagation is on the hot path (called every Lambda invocation).
• A follow-up PR in bottlecap will add datadog-opentelemetry as a dependency and replace its local propagation code.

[iunanua]

👍 LGTM