[webview_flutter_android][webview_flutter_wkwebview] Adds platform implementations for onHttpError

[bparrishMines]

Copy of #3695 since it doesn't contain permission to edit from contributors.

Part of flutter/flutter#39502

Full PR #3278

Pre-launch Checklist

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[stuartmorgan-g]

LGTM with minor feedback.

I do worry that doing the on-response-to-on-error mapping on the Dart side on iOS is going to make every load operation slower, but it's likely that the native->Dart->native round trip will be negligible compared to web load times. If it becomes a problem in practice, we'll need to revisit this and flex the API surface to do the response code checks on the native side and make it an on-error callback at the Pigeon protocol level.

[jmagman]

@bparrishMines is this ready to merge, or are you waiting for more reviewer feedback?

[bparrishMines]

It's ready, I just forgot about adding the label. I also wanted to respond to @stuartmorgan about including a is<FunctionName>Overriden feature in the pigeon wrapper once the initial implementation is done.