dd-trace-cpp: Improper parsing of W3C baggage headers may lead to DoS

dd-trace-cpp is the Datadog distributed tracing library for C++. Prior to 2.1.0, dd-trace-cpp parses incoming W3C baggage headers without enforcing DD_TRACE_BAGGAGE_MAX_ITEMS or DD_TRACE_BAGGAGE_MAX_BYTES on the extraction path, even though those limits are enforced during injection. A remote unauthenticated attacker can send a header containing many comma-separated key-value pairs or one very large value, causing per-request hash-map allocation and unbounded CPU and memory consumption. Baggage extraction is enabled by default in most affected tracers unless baggage is removed from DD_TRACE_PROPAGATION_STYLE or DD_TRACE_PROPAGATION_STYLE_EXTRACT, so affected internet-facing services can be denied service. This issue is fixed in version 2.1.0.