Allow setting ENV vars both in apprunner.yml and web console

It is sometimes required to set ENV variables only for a given deployment, and not for the entire application. An example is the `redirect_uri` setting for typical OAuth2 authentication servers, where the full URL is required and the domain for the apprunner is randomly generated. In such a system, there is no point in saving it in `apprunner.yml`, because the same file can be used for several services.

Alternatively, add an environment variable such as HOSTNAME to Apprunner that holds the app's auto-generated URL and is available at runtime.

No source files, tests, or entry points are named because this is a product-roadmap request rather than an implementation task. Start by clarifying whether per-deployment web-console variables, an App Runner HOSTNAME variable, or both are required; done would mean an agreed behavior and documented deployment/runtime semantics.

Written by the indexing model from the issue text.