feat: use trace-context "headers" in SNS and SQS single event message…

… attributes for lambda transaction

- If a SNS or SQS single event trigger to an instrumented Lambda
  function includes message attributes with the name "traceparent" (and
  "tracestate"), case-insensitive, then those are used to continue the
  trace. This was already being done for API Gateway event headers.
- Also, fix a bug in the capturing of SNS and SQS message attributes as
  "transaction.context.message.headers". Note that only "String"-type
  message attributes are captured. Binary-type attributes are base64
  encoded. I'm not sure of the value in capturing them.

Closes: #1831
Fixes: #2605
Refs: elastic/apm#614