POC: Create transaction in lambda extension and record error results

[simitt]

This enhancement issue to build a POC to investigate creating transaction events on behalf of the Lambda function, and report an error result (e.g. "OutOfMemory") in transaction.result in case an error occurs.

Related go agent work: elastic/apm-agent-go#1323

** Needs more definition **

[axw]

My expectation is that eventually, the transaction document will capture:

• the error result, e.g. OutOfMemory or TimedOut
• Lambda/faas attributes and metrics

The document would look something like

{
  "transaction": {
    "name": "...",
    "result": "OutOfMemory"
  },
  "faas": {
    "execution": "...",
    "id": "...",
    "coldstart": true,
    "duration": 123.456,
    "billed_duration": 200,
    "coldstart_duration": 100,
    "timeout": 5000,
  },
  "system.memory.total": 536870912,
  "system.memory.actual.free": 0,
}

Later, the faas.* and system.* fields will be aggregated in transaction metrics. This should ideally be generalised to request-scoped metrics.