Move iOS codesigning into assemble build target - #77664

1. Pass the codesigning identity into flutter assemble debug_ios_bundle_flutter_assets.
2. Move App.framework codesigning out of xcode_backend thinning and into the IosAssetBundle (debug_ios_bundle_flutter_assets etc) assemble target as the last step of building that framework. This allows codesigning to be skipped if the App.framework doesn't need to be rebuilt.
3. Move Flutter.framework codesigning out of xcode_backend and into the UnpackIOS (debug_unpack_ios etc) assemble target as the last step of copying around that framework.
4. Flutter.framework is codesigned with Headers and Modules directories (necessary for linking) which means those directories can't be removed during embedding, or the codesigning validation on install will fail. So stop removing those directories from the final embedded app (they are really small).

Confirmed when the code signing identity changes, the frameworks are re-signed (because the assemble inputs changed).

Added unit tests, but also covered by ios_content_validation_test.

Second compilation getting faster (see #77420):

$ dart bin/run.dart -t hello_world_ios__compile
...
    "debug_full_compile_millis": 18347,
    "debug_second_compile_millis": 9663

Part of #67580
See also #77329 and #77007