Automating certificate creation for custom domains in Container Apps

### Is there an existing issue for this?

- [x] I have searched the existing issues

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

Currently Aspire has `ConfigureCustomDomain` for ACA, but the problem is that you can't specify both the custom domain and the managed certificate at once ("chicken and egg" situation described in #6271).

So `ConfigureCustomDomain` doesn't create the manage certificate, it can only reference an existing one which leaves 2 options:
- Creating it manually
- Parametrizing the rollout and adding a custom Bicep step that creates it

### Describe the solution you'd like

Bicep recently added a `@nullIfNotFound` decorator to `existing` resources. Aspire could generate a Bicep module with an output indicating whether the container app has been created, and accordingly create the certificate and reference it in the app. So in order to fully complete the process, we'd only need to rerun the same deployment after a first successful run.

### Additional context

_No response_