Allow for different send strategy via environment variable

[estolfo]

Resolve #75

[ghost]

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• Start Time: 2022-01-13T10:13:51.671+0000

• Duration: 7 min 54 sec

• Commit: 4954fe6

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[astorm]

👍 does the thing it says it does and fulfills the request in #75

I'd say the only thing left is getting our new env variable added to the docs at https://github.com/elastic/apm-aws-lambda/blob/main/docs/aws-lambda-extension.asciidoc#lambda_env and we can call this one good and merge it.

[estolfo]

thanks for the reminder about the docs!

[estolfo]

@astorm Updated the PR with documentation for the new env variable 👍🏼