Pipeline Manifest Json

### 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.

I am trying to create a UI for the Aspire pipeline, and I am currently parsing the `aspire do diagnostic` human readable output, skipping the logging prefix, and regexing the data into a structure so that I can use it.

### Describe the solution you'd like

I'd like to be able to output into a file / stdout the manifest of the pipeline - which steps are defined, what resource (if any) defined it, etc.

I'd like to be able to call the aspire cli with something like `aspire do diagnostic --format json` and get the manifest.

If the manifest could have the cmd being executed for steps that are simple process calls then that would be even better.

### Additional context

This POC works: https://github.com/tjwald/aspire-pipeline-viewer
But if you change the format even a bit it will break.