Add CI job to check iOS/macOS symlinks [SDK-4714] 1/2

[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 ensures the iOS/macOS symlinks are always in sync with the contents of the darwin directory, by adding a new CI job that:

1. Regenerates the iOS/macOS symlinks
2. Uses Git to check if there were any changes

If there are changes, the CI job fails and prints a message informing how to fix the problem. If there are no changes, that means the symlinks are in sync, so the job succeeds.

[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@844e03c). Click here to learn what that means.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feature/macos-support #352 +/- ## ======================================================== 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/352/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/352/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/352/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/352/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/352/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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