Add tests that validate that playground app manifests are not broken

[joperezr]

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@mitchdenny suggested that it would be good if we add a set of tests to the repo that grab the playground apps and generates manifests for all of those, and that it would then compare them against a checked-in version of them to ensure we catch any unexpected breaks in manifest generation. This issue is to track that work.

[mitchdenny]

@radical is handling this as part of another issue/PR:

https://github.com/dotnet/aspire/pull/5208

[joperezr]

@radical @mitchdenny is anyone of you still planning on working on this in the next couple of weeks? If not, let's push it to next month.

[mitchdenny]

Putting it to the backlog. @radical has done work on running playground apps during CI - but I don't think we are doing the manifest check yet.