[iOS] separate storage provider

[kambala-decapitator]

Description

Splits DarwinStorageProvider into OSX and iOS implementation and also adds free/total space display to iOS.

Motivation and Context

iOS didn't show anything in System info - Storage.

How Has This Been Tested?

• built project for arm64 and ran on iOS device
• verified that macOS code didn't become broken

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[kambala-decapitator]

Also a question here: would it be better to use already existing functions to get disk space: CSysInfo::GetDiskSpace (returns space in MB including used space and percents) or CSysInfo::GetHddSpaceInfo (already provides formatted strings, but requires calling it multiple times to get all the info)?

[kambala-decapitator]

Jenkins build this please