Add `AlbHealthCheckLayer` (#2540)

## Motivation and Context
Services often need the ability to report health status via health
checks (see [ALB Health
Checks](https://docs.aws.amazon.com/elasticloadbalancing/latest/application/target-group-health-checks.html)).
This PR adds a simple layer that allows configuring your service to
respond to these health check requests.

## Description
Adds `AlbHealthCheckLayer`, and `AlbHealthCheckService`.

## Testing
Added this layer to the `pokemon-service` binary, and added an
integration test for it.

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