Jetpack: add Sign In With Apple

[leandroalonso]

This PR adds the SIWA button/flow on the Jetpack app.

Keep in mind that in order to fully test this flow it will need:

1. To download this app version from TestFlight
2. To submit and deploy some small changes on the backend

To test

1. Make sure the CI is all green

@jkmassel let me know if you have a better idea on how to test this PR.

Regression Notes

1. Potential unintended areas of impact
   n/a

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   n/a

3. What automated tests I added (or what prevented me from doing so)
   n/a

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.