Add support for reloadable process types RFC

[dmikusa]

Summary

• If BP_LIVE_RELOAD_ENABLED is set and true
  • the CNB will require watchexec
  • the CNB will add a process type called reload that calls watchexec -r <normal command>
  • the CNB will set the default process type to be reload

This can be used in conjunction with a Tiltfile like:

custom_build(
   'example-java-image',
   'pack build --pull-policy=if-not-present -e BP_LIVE_RELOAD_ENABLED=true -e BP_GRADLE_BUILT_ARTIFACT=build/distributions/*-boot-*.zip',
   ['pom.xml', 'build/libs/demo-0.0.1-SNAPSHOT.jar'],
   live_update = [
       sync('build/libs/demo-0.0.1-SNAPSHOT.jar', '/workspace/demo-boot-0.0.1-SNAPSHOT/lib/demo-0.0.1-SNAPSHOT.jar'),
   ],
   tag="dev"
)
k8s_yaml('kubernetes.yaml')
k8s_resource('example-java', port_forwards=8000)

When code changes are made to a watched directory, Tilt will push the changes out to the container. Those will be observed by watchexec and the main process will reload, reloading your changes. With DistZip this is a little bit less practical because you are pushing up a ZIP file that contains a JAR file with all of your application files in that. You cannot easily recompile and upload a few class files. It is technically possible though to recompile and send your entire JAR file. When done, watchexec will see the change and reload the application.

Use Cases

Implement the Reloadable Process Type RFC

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).