feat: Allow plugins to override navigation

[jcesarmobile]

variation of #2307

changes:
Android method renamed from shouldOpenExternalUrl to shouldOverrideLoad (more similar to the Android WebView method, and matches the iOS name). Logic changes, false load, true abort, null default.

ios method renamed from shouldOverrideLoadWith to shouldOverrideLoad, with a single WKNavigationAction param instead of two (you can get both request and navigationType from it).
Method return type changed from Bool to NSNumber so it can return nil.
And as it can return nil now, change logic to work as on Android (false load, true abort, nil default)

closes #2307
closes #70

[sbannigan]

Looks good to me 👍

[imhoffd]

Only one plugin can do this in an app, right?

[jcesarmobile]

There can be multiple plugins, but totally depend on them, if they only handle certain url and return null/nil for other cases, then another plugin can take care of other URLs, if they return true or false for all URLs then other plugins won’t be able to handle them as it gets the value from the first plugin that returned true or false.

[imhoffd]

Gotcha.