Support for `AWS_LAMBDA_EXEC_WRAPPER`

**Is your feature request related to a problem? Please describe.**

@Datadog's ASM support for Lambda is implemented using a proxy around the `AWS_LAMBDA_RUNTIME_API` so we can intercept requests & responses and pass them to the WAF (for monitoring and possibly blocking).

This flow requires replacing the value of `AWS_LAMBDA_RUNTIME_API` with an alternate endpoint, which is normally achieved by setting the `AWS_LAMBDA_EXEC_WRAPPER` environment variable on the Lambda function to `/opt/datadog_wrapper` which then sets `AWS_LAMBDA_RUNTIME_API` to the `host:port` of the reverse proxy started by the Datadog extension.

Unfortunately, the provided runtimes (`provided`, `provided.al2`, and coincidentally `go1.x`) do not actually honor this environment variable and require special casing.

**Describe the solution you'd like**

It's unclear why custom/provided runtimes fail to honor this setting, and we would like to understand if it would be possible for the provided runtimes to actually do this.

Failing that, we think it would be feasible (although perhaps not desirable, notably because of the added cold start latency) for `github.com/aws/aws-lambda-go` to come with an `init` function that detects the `AWS_LAMBDA_EXEC_WRAPPER` environment variable, and when present, uses `syscall.exec` to enact the other runtime's flow.

**Describe alternatives you've considered**

We are considering a focalized solution in https://github.com/DataDog/datadog-lambda-go/pull/143, but feel like this would be best addressed higher up in the dependency/responsibility chain.

**Additional context**

See: https://docs.aws.amazon.com/lambda/latest/dg/runtimes-modify.html#runtime-wrapper.
See: https://github.com/DataDog/datadog-lambda-extension/blob/main/scripts/datadog_wrapper.