chore: add migration guide from AWS SDK for iOS to Amplify Swift

[harsh62]

This pull request adds a comprehensive migration guide for transitioning from the AWS Mobile SDK for iOS to Amplify Swift, addressing various AWS service categories and providing detailed instructions, comparisons, and resources.

Issue #

Description

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.