feat: add support for Lambda data flushing

[trentm]

This adds 'Client#lambdaStart()' and the 'lambdaEnd: true' option to 'Client#flush([opts,] [cb])' to support for flushing Lambda invocation tracing data and signaling the Elastic Lambda extension per spec. https://github.com/elastic/apm/blob/main/specs/agents/tracing-instrumentation-aws-lambda.md#data-flushing

Refs: https://github.com/elastic/apm-agent-nodejs/issues/2485

Checklist

• [x] waiting for extension panic fixes before enabling the '?flushed=true' signaling by default
• [x] tests of the new APIs

[apmmachine]

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#### Build stats * Start Time: 2022-03-04T00:58:23.109+0000 * Duration: 9 min 55 sec #### Test stats :test_tube: | Test | Results | | ------------ | :-----------------------------: | | Failed | 0 | | Passed | 287 | | Skipped | 0 | | Total | 287 |

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[trentm]

elastic-apm-http-client@11.0.0 is published