Overhaul heroku-deploy-standalone

[Malax]

This PR overhauls heroku-deploy-standalone. Main objectives were to:

• Improve UX.
• Provide in-line help in failure scenarios.
• Align feature set with build tool Heroku deploy plugins (such as heroku-maven-plugin).
• Improve security by automatically using the latest webapp-runner instead of a fixed version.
• Add a basic suite of integration tests.

This is just the first PR in a sequence for heroku-deploy-standalone improvements and eventual sunset of heroku-maven-plugin.

PR roadmap:

• Windows integration tests.
• Extended integration tests for more advanced use-cases.
• Improved README.
• Removal of heroku-deploy, heroku-maven-plugin and code that is no longer necessary after removal of the former modules.
• Support for Heroku-Deploy-Type header.
• Maybe: Caching of Rust dependencies in CI.

Review notes:

Code changes to the CLI are in commit 2e4df22. Integration tests in 36b7a35. Both commits can be reviewed independently.

Ref: GUS-W-14462146, GUS-W-14462149