[SIWA] Log In: add 'Continue with Apple' button to login options

[ScoutHarris]

Fixes #n/a
WPAuth PR: wordpress-mobile/WordPressAuthenticator-iOS#113

This enables the new enableSignInWithApple flag added in the above WPAuth, allowing the 'Continue with Apple' button to appear in the Log In button view.

To test:

• Run the app in XCode 11.
  • Run on iOS 13. Verify the Apple button appears.
    • Tapping the button doesn't do anything yet, but does log a Login Prologue: Apple tapped. message.
  • Run in a lower iOS version. Verify the Apple button does not appear.
• Run the app in XCode 10.
  • Verify the Apple button never appears.
• Disable FeatureFlag:signInWithApple.
  • Verify the Apple button never appears.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[astralbodies]

Works as expected! That post_install hook scares me a little.