Support AWS Lambda behind Load Balancer (ALB)

[zoellner]

Describe the feature

The current aws-lambda present assumes that the lambda is behind an API Gateway (tries to distinguish between v1 and v2). But there are other infrastructure configuration options and it is possible to have the lambda behind an ALB (see https://aws.amazon.com/blogs/networking-and-content-delivery/lambda-functions-as-targets-for-application-load-balancers/)

The event format is slightly different in that case. ALB can also have a multi value header option turned on or off (https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html#enable-multi-value-headers) which changes the event format as well.

Related to #505 (split presets into different one for these use cases) and #79 (lambda-edge support)

This repo does a good job at distinguishing between the different event formats and normalizing it: https://github.com/dougmoscrop/serverless-http

Additional information

• Would you be willing to help implement this feature?

[pi0]

Moving to the unified tracker: #2973

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• Please comment here if you would like to work on it