* Add `onScroll` callback for iOS & Android

This code was mostly extracted from https://github.com/react-native-community/react-native-webview/pull/202

I tried and tried to make it work with `Animated.event`'s `useNativeDriver`, but I was unsuccessful 😢 that'll have to be done later once I understand better how Animated's native stuff is hooked up.

* fix crash for missing onScroll

* [iOS] Allow focus without user interaction

* Add documentation for keyboardDisplayRequiresUserAction

* set keyboardDisplayRequiresUserAction default to true

We had the problem on iOS WebViews that local cookies (stored in local HTTPCookieStorage, set with  [react-native-cookie](https://github.com/shimohq/react-native-cookie) ) were not added in loadRequests. On Android the local stored cookies were sent like expected.
This kinda "hacky" solution is the only way we found, that works for us.
The stack overview link is in the code below.
If someone finds a better solution we would very much like to accept that.

Added a new cacheEnabled prop to toggle Android & iOS webview caching behavior.

BREAKING CHANGE:  This change makes caching enabled by default when previously there was no caching behavior which may cause unexpected behaviour changes in your existing implementations.

Allows the webview to be opened with an ephemeral data storage.

* fix(WKWebview): [iOS] Add shared process pool so cookies and localStorage are shared across webviews (#68)

* Add optional shared process pool

BREAKING CHANGE: useSharedProcessPool prop is set to true by default. If you want the old behavior, please use useSharedProcessPool={false}

* [ios] Add ‘allowsLinkPreview’ property to iOS WKWebView.

* Add divider in docs

* Typo: add hyphen -> _allowsLinkPreview

* Add 'userAgent' property to the iOS WKWebView

* Update 'userAgent' reference docs.

* add hideKeyboardAccessoryView option

* add hideKeyboardAccessoryView prop to reference

* Added the onLoadProgress property to the iOS wkwebview and Android webview