fix(@angular-devkit/build-angular): serve assets

[jkrems]

Turns out the test for asset serving didn't actually test asset serving because of a missing whenStable() - it always succeeded, even with random filenames. This change adds a custom middleware to serve our static files since it seemed easier (and more predictable) than to try to map the assets concept to Karma's files+proxy logic.

One side effect of adding the middleware is that we don't actually need the Karma files anymore with the exception of any files that we'd want to include on page load. I don't touch that part here because it will be cleaner to change it when also addressing the naming collisions between spec files.

Fixes #28754

P.S.: Currently the first commit is from a different PR (#28794), I'm planning to rebase after landing it.

[jkrems]

The changes were merged into the following branches: main, 19.0.x

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