Support getting information from AWS lambda (remaining time, etc.)

[koterpillar]

As in native runtimes, the environment information must be available to the process upon request.

[dmoverton]

The AWS Lambda custom runtime API returns this information as response headers. See https://docs.aws.amazon.com/lambda/latest/dg/runtimes-api.html#runtimes-api-next.
We could use these to construct a Context object to pass from lambdaMain to the handler function as an extra parameter, as is done with several of the supported language runtimes.