feat(auth): Removed dependency on oauth2 token for refreshToken and move to initiateAuth for hostedUI

[harsh62]

Issue #

#3496

Description

Currently we use oauth2/token endpoint to refresh tokens when the user is logged in via HostedUI. The premise of this was you need to refresh using the hostedUI api to preserve custom scopes. Service team has confirmed that we do not need to call the hosted UI endpoint api for refreshing of the tokens and simply calling the initiateAuth api will also preserve custom scopes as long as the sign in was done using the HostedUI api.

Furthermore, this brings us in parity with other platform implementations. Android and Flutter already does refreshing of tokens this way.
Android PR: https://github.com/aws-amplify/amplify-android/pull/2543/files

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.