Extension - Periodically health check the Contained App

[huntharo]

Motivations

• Currently we only check the app health during init
• If the contained app exits / closes its listening port we need to exit the exec env since we cannot recover from this

Acceptance Criteria

• Add a test that allows the initial health check to succeed but then stops responding to the healthcheck and have this test fail because it does not return an error from the LambdaService
• Add periodic polling of the healthcheck route
• Add settings to control the healthcheck route, interval, and timeout
• Update documentation