af: Gate native Swift/Objective-C code per platform [SDK-4700] 1/3

[Widcket]

• [ ] All new/changed/fixed functionality is covered by tests (or N/A)
• [ ] I have added documentation for all new/changed functionality (or N/A)

📋 Changes

This PR gates all the iOS-specific code and macOS-specific code in auth0_flutter behind compile-time conditionals, so it compiles correctly on both platforms.

Also the Podspec of auth0_flutter was updated to use the FlutterMacOS dependency on macOS, instead of Flutter, which is iOS-only, and the minimum deployment target of the sample app was updated to match the version specified in the Podspec.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:exclamation: No coverage uploaded for pull request base (feature/macos-support@c5ee3e7). Click here to learn what that means.

:exclamation: Current head bb1bfbf differs from pull request most recent head 9303d01. Consider uploading reports for the commit 9303d01 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feature/macos-support #346 +/- ## ======================================================== Coverage ? 96.36% ======================================================== Files ? 98 Lines ? 1596 Branches ? 328 ======================================================== Hits ? 1538 Misses ? 50 Partials ? 8 ``` | [Flag](https://app.codecov.io/gh/auth0/auth0-flutter/pull/346/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | Coverage Δ | | |---|---|---| | [auth0_flutter](https://app.codecov.io/gh/auth0/auth0-flutter/pull/346/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `100.00% <0.00%> (?)` | | | [auth0_flutter_android](https://app.codecov.io/gh/auth0/auth0-flutter/pull/346/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `97.35% <0.00%> (?)` | | | [auth0_flutter_ios](https://app.codecov.io/gh/auth0/auth0-flutter/pull/346/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `99.85% <0.00%> (?)` | | | [auth0_flutter_platform_interface](https://app.codecov.io/gh/auth0/auth0-flutter/pull/346/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `87.03% <0.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0#carryforward-flags-in-the-pull-request-comment) to find out more.

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[Widcket]

The flutter analyze CI check (CI / Analyze auth0_flutter_platform_interface Flutter package) will pass once https://github.com/auth0/auth0-flutter/pull/347 gets merged.