Expose Webview component

[airandfingers]

We'd like to customize our Webview's settings, including its user agent string and permissions. On Android, these require access to the Webview component, to make calls like these:

let settings = webView.android.getSettings();
settings.setUserAgentString('My Custom UA String');
settings.setAllowFileAccess(false);

nativescript-oauth doesn't currently expose the Webview component, so we currently can't change settings like this.