[pigeon] Add local-only Kotlin integration tests (#2852)
* [pigeon] Add local-only Kotlin integration tests
Adds a new test target to run the new shared integration tests on
Kotlin, and adds the necessary native-side plumbing.
This is not run by default because it requires a device or emulator to
be present, and we don't have that set up for CI yet (and we don't yet
have a different script entrypoint for manual vs CI tests).
Enabling this in CI is tracked in
https://github.com/flutter/flutter/issues/111505, but landing it now
allows for better manual testing of generator changes during development.
Part of https://github.com/flutter/flutter/issues/111505
* Add copyright
[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
## Pre-Review Checklist
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[pigeon] Timestamp test steps in CI (#8716)
Pigeon's tests are taking a substantial amount of CI time, so it would be useful to be able to audit how long each step takes so we have more information if we need to shorten or shard these tests.