Please discourage custom schemes

I'm an engineer on the Android WebView team. Custom schemes are poorly defined in web platform standards, and Android WebView generally struggles to maintain backwards compatibility for these. #348 and https://issuetracker.google.com/issues/140057007 are just a couple examples of this fragile use case breaking.

I see https://github.com/ionic-team/cordova-plugin-ionic-webview#scheme documents support for changing the scheme of content. While we encourage changing the scheme to `https://` (and would even encourage making this the default, for improved web security), we strongly discourage any other scheme (`data:`, `file:///`, `blob:`, custom scheme, etc.). I would advise your docs to show the same recommendation.

If the app wants to distinguish injected content from real web content, they should host it on a unique domain rather than a unique scheme. This is the intended use case for https://developer.android.com/reference/androidx/webkit/WebViewAssetLoader.Builder.html#setDomain(java.lang.String).

CC @HazemSamir (the author of WebViewAssetLoader).