feature: handle MSK events

[joeyzhao2018]

https://datadoghq.atlassian.net/browse/SLES-2739

In Kafka's wire protocol (KIP-82), header values are always byte[]. Every Kafka client library enforces this:

Tracer	Injection code	Mechanism
dd-trace-java	headers.add(key, value.getBytes(UTF_8))	String.getBytes() → byte[]
dd-trace-go	Value: []byte(val)	Go type conversion → []byte
dd-trace-dotnet	_headers.Add(name, Encoding.UTF8.GetBytes(value))	UTF8.GetBytes() → byte[]

All three tracers accept string trace context values from the propagation layer, convert to UTF-8 bytes at the carrier adapter boundary, and hand byte[] to the Kafka client.
This isn't a quirk of Java's getBytes() — it's the only way Kafka headers work.

What MSK Lambda does

When MSK triggers a Lambda, AWS serializes the Kafka record to JSON. Since header values are byte[] on the wire, AWS encodes them as decimal byte values. However, the exact JSON
shape depends on the Lambda runtime:

• Array format (observed in the existing msk_event.json testing payloads, i didn't change the support for this to be safe): byte values as a JSON array of integers
  "headers": [{"x-datadog-trace-id": [51, 54, 57, ...]}]
• Object format (observed with the Java Lambda runtime): both the records list and the per-header byte values are JSON objects with numeric string keys, and byte values are
  decimal strings
  "records": {
  "topic-0": {
  "0": {
  "headers": {
  "0": {"someOtherHeader": ["70", "114", ...]},
  "2": {"x-datadog-trace-id": {"0":"52","1":"54",...}},
  "4": {"x-datadog-sampling-priority": ["49"]}
  }
  }
  }
  }
• Note that Datadog headers can appear at any index — non-instrumentation headers may precede them.

What's the difference between the msk_event.json and the newly added msk_event_with_headers.json here?

• msk_event.json represents a standard MSK trigger where the producer didn't attach any Kafka headers — i.e. no Datadog tracer was running on the producer side (or it's a non-instrumented producer like a raw Kafka client, a Kinesis Firehose delivery stream, or a schema-registry message). In those cases Lambda still delivers the event but with "headers": []. It's also the format you get when testing MSK triggers manually in the AWS console, which doesn't inject headers. ( source: Claude Code)
• msk_event_with_headers.json reflects the real-world object format produced by the Java Lambda runtime, with a producer instrumented with a Datadog tracer injecting trace context
  as Kafka headers. It includes non-Datadog headers at lower indices to verify that the carrier extraction correctly finds Datadog headers regardless of their position. (source: I did a real world example and below is the evidence of testing)