feat(ios): add InAppBrowserStatusBarStyle 'darkcontent' configuration option

[NiklasMerz]

Platforms affected

iOS

Motivation and Context

If the IAB loads a page with a white background on a device with dark mode enabled the statusbar will be white with white content. This PR ads the config.xml to set statusbar content to dark to make it readable again for pages like this.

Description

#765 added the option to set the statusbar to light content. iOS 13 introduced a new dark content option and this PR adds that.

Testing

Manual test
No automatic tests

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[jcesarmobile]

We should be careful with the @available usage, the code inside it is not understand if using an older SDK to build than the version on the @available, which means, this code doesn't compile on Xcode 10 or older.

For this PR is not a big deal since we are planning a major release and can bump the engines to cordova-ios 6 instead of 5.1.0 as on my pending PR, but then noticed version 4.1.0 is already using @available with iOS 13 and fails to build.

I've reported the issue