[pigeon] Convert test plugins to SPM (#9105)
Updates the test harness plugins to support SPM, so that tests won't rely on CocoaPods going forward.
For now I've left CocoaPods support in the plugins and the integration in the example app, so that local tests won't rely on SPM support being on, but in the future when needing to disable it locally for testing plugins is rare we can fully remove CocoaPods from the test plugins and their apps since they aren't public plugins and thus don't need to support both modes once we don't use both ourselves.
Part of https://github.com/flutter/flutter/issues/146922
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