Support for environment variables

Is your feature request related to a problem? Please describe.

I would like to have an out of the box supported standard approach to use environment variables to configure my middlewares and tasks.

Why? As one of the maintainers of the middleware in the open-ux-tools, I have noticed a repetitive pattern: usage of environment variables. In nearly all our middlewares/tasks we support some kind of usage of environment variables as input. A quick search through other middlewares also shows that we are not the only ones but many others also have their way of allowing to provide parts of the configuration as env variables.
The problem is that everybody does their own thing leading to inconsistencies of how to use env variables.

Describe the solution you'd like

I'd prefer a very simple solution allowing me to define environment variable placeholders in my yaml e.g.

server:
  customMiddleware:
  - name: preview-middleware
    afterMiddleware: compression
    configuration:
      debug: ${DEBUG}

The UI5 CLI should resolve the environment variable before calling my middleware so that the middleware implementation does not have to bother.
Please also support dotenv out of the box as part of the same feature.

Describe alternatives you've considered

No.

Additional context

No.

No files or tests are named. Start by tracing how the UI5 CLI reads YAML configuration and passes middleware and task configuration, then determine where environment-variable and dotenv resolution belongs; done means placeholders are resolved before middleware or task execution with dotenv supported out of the box.