Rethinking the iCloud sign-in flow

[dedoussis]

Instead of signing-in through the pop-up, the extension will prompt the user to sign-in through https://icloud.com. The background script will sniff the responses of the icloud.com requests (via webRequest listeners) to extract the auth tokens it needs.

Signing-in through the official icloud.com page of Apple has the following benefits:

1. Support for security key 2FA (alongside the 6 digit code one). Fixes https://github.com/dedoussis/icloud-hide-my-email-browser-extension/issues/41 and maybe https://github.com/dedoussis/icloud-hide-my-email-browser-extension/issues/26
2. Support for Passkeys (alongside passwords)
3. Feels more secure from a user pov. Even though there's no practical difference, users feel more comfortable submitting their password on an Apple hosted surface rather than a random extension.

TODOs to get the extension into a publishable state:

• [x] Add post installation help page
• [x] Test 6 digit code 2FA
• [x] Test passkey sign-in
• [x] Test on Firefox
• [ ] Fix dependabot alerts
• [x] Stretch: rework the SignInInstructions copies/presentation