feat: make buffer size for agent data configurable

[kruskall]

Add ELASTIC_APM_LAMBDA_AGENT_DATA_BUFFER_SIZE variable to set
the buffer size.

Closes #28

[ghost]

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• Start Time: 2022-08-03T20:18:20.849+0000

• Duration: 7 min 49 sec

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[axw]

Looks good, just one nit.

I think we should also update the docs: https://github.com/elastic/apm-aws-lambda/blob/main/docs/monitoring-aws-lambda.asciidoc#configuration-options

[kruskall]

Updated the documentation and added a changelog entry.