feat(rtn-passkeys): add ios support - 3a

[jjarvisp]

Description of changes

The purpose of this PR is to add iOS support for the React Native implementation of webauthn.

• Adds objc adapter class to map typed native input from JSI to swift
• Adds swift class containing complementary getPasskey, createPasskey, and getIsPasskeySupported APIs
• Adds swift delegate for handling async UI task completion to return assertion and registration result
• Adds helper utility for transcoding base64url
• Adds corresponding mock data and unit tests for swift implementation

Issue #, if available

Description of how you validated changes

• manual tests
• unit tests: https://github.com/aws-amplify/amplify-js/actions/runs/15143765044/job/42574552126

Checklist

• [X] PR description included
• [X] yarn test passes
• [X] Unit Tests are changed or added
• [ ] Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• [ ] Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• [ ] New source file paths included in this PR have been added to CODEOWNERS, if appropriate

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