Support partial access permissions for iOS Photo Library operations

[antoinevg]

iOS 14 brings the option of only providing permission for some items in the user's Photo library:

As of iOS 14 Beta 4 selecting this option does not return a valid PHAsset object for requested items. Even when permission was granted for those items.

What's also unknown is whether Apple are providing an interface that allows the user to change their mind about which items they want to allow access to in subsequent photo library interactions.

More information:

• https://developer.apple.com/documentation/photokit/phauthorizationstatus?language=objc
• https://developer.apple.com/documentation/photokit/phphotolibrary/3616053-requestauthorizationforaccesslev?language=objc

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• [x] Remove iOS permissions code for forge.file and forge.gallery (Can we do the same for Android?)
• [x] Only ask for add permissions for iOS forge.capture (Can we do the same for Android?)
• [x] Remove permissions no longer needed for iOS forge.permissions implementation
• [x] Remember to update documentation!

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• [x] file module
• [x] capture module
• [x] gallery module
• [x] permissions module

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• [x] migration guide
• [x] permissions module documentation