ohos: Port HarmonyOS WebView support to Qt6 dev branch

Port qtwebview HarmonyOS backend from a private 5.15.16 fork
to the dev branch.

The following QWebViewPrivate members are not yet implemented
due to structural differences between Qt5 and Qt6:

- setCookie()
- deleteCookie()
- deleteAllCookies()

QWebViewSettingsPrivate is also not implemented, as this
class did not exist in Qt5 and requires a new implementation
for Qt6.

Task-number: QTBUG-146838
Pick-to: 6.12
Change-Id: I1a9fe0a4b81c7c58909bbe64152d3efae335b52a
Reviewed-by: Liang Qi <liang.qi@qt.io>