lambda: trace-context trace continuation for SNS and SQS single event triggers

[astorm]

Once we take care of #1521, we'll want to make sure we add tracestate header parsing to the lambda transaction starting code.

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Update (by trentm): I realize belatedly that this issue had been about extending the initial handling of the traceparent header in the Lambda instrumentation to also handle the tracestate header. That was done in #2185. I have morphed this issue to be about handling those trace-context headers for the SNS and SQS trigger cases.

Note (by trentm): The commit for elastic/apm-nodejs-http-client#180 reference this issue. It should have referenced #2598 instead.

[trentm]

Trace context is being picked up from event.headers to use as childOf for creating the lambda transaction. However there is some remaining work here:

• The current event.headers checking is being done for all trigger events, and not scoped to API Gateway events.
• Possible traceparent/tracestate from SNS and SQS single-event record.messageAttributes is not being picked up, but it is part of the spec.
• We don't have test cases for any of the above yet.