[ios][storage] Add guide for uploading security scoped resources

[atierian]

Is your feature request related to a problem? Please describe. Uploading files stored in the iCloud Drive is a common use case. Amplify currently doesn't have any guides on how to properly handle security scoped resources (in iCloud Drive).

Describe the solution you'd like Add a guide explaining how to properly access and upload security scoped resources.

Describe alternatives you've considered N/A

Additional context Original issue that inspired this issue, including a (simplified) solution.