Update to provided.al2 runtime (Go 1.x runtime deprecated by AWS)

[drocco007]

Hi. AWS is deprecating the Go 1.x Lambda runtime and requiring Go-based Lambdas to use the provided.al2 runtime instead.

There are a handful of minor snags to migrating this project:

• glibc library version mismatch between the development and runtime environments. The Goreleaser config file already disables CGO (build: disable CGO #12), so it seems safe to do the same in the makefile.
• the provided.al2 runtime requires a specific executable name: the handler executable must be called bootstrap
• the runtime also requires bootstrap to be placed in one of two specific locations: either /var/run/task (the Lambda package itself) or /opt (a Lambda layer). As currently packaged, the executable is placed in a bin/ subdirectory, which the runtime will not execute, even if the Lambda configuration attempts to reference it exactly.

For the last problem, I tested out the idea of using the makefile to generate the zip file — without the bin/ prefix — and then using the artifact attribute in the serverless config to have the deploy step use that zip instead of packaging up a new one. Alternatively we could have make dump the bootstrap binary directly into the project root and let serverless continue to handle packaging.