[Feat] keep webdriverAgent active in Background, for clipboard ability

[wabicai]

Over iOS 14+ version, we could not run driver.getClipboard in real iOS device, for secure reason.
"app could not get Clipboard data when it's running background"

So I thought a method to get clipboard while webdriver running background.
See the apple office document:
https://developer.apple.com/documentation/xcode/configuring-background-execution-modes

I think it may be helpful, if webdriver could keep active in background.

[KazuCocoa]

Did you confirm that the method helped? I wondered if it could increase background resource usage than without that. Then, we could probably leave it as an advanced guide. Perhaps it requires a new provisioning profile.