core: add BackendApplicationServer

[paul-marechal]

The BackendApplicationServer is an optional backend contribution that
serves frontend files. When not bound, the generators may bind one.

This component is useful when your application is packaged in such a way
that you need to customize how the frontend is served.

How to test

1. Run the browser app example, the frontend should load like before.
2. Shutdown the backend.
3. Set an env var such as: SAMPLE_BACKEND_APPLICATION_SERVER=1
4. Run the browser app example again, it should display SampleBackendApplicationServer OK

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

The changes look good to me and work well.

When setting the SAMPLE_BACKEND_APPLICATION_SERVER=1 environment variable, the application restarts with the "SampleBackendApplicationServer OK" page.