SIWA: Update the login return uri

[Tug]

Changes proposed in this Pull Request

• This PR updates the login return url for sign in with apple from POST /log-in/apple/callback to POST /sign-in/apple/callback.
• It also updates how we parse the JSON user object coming from apple as there seems to have been some changes around this.

Testing instructions

• Run this branch locally npm start
• Disable your a11n proxy and proxy https://wordpress.com to http://calypso.localhost:3000
• Try signing in with apple to an existing account from https://wordpress.com/log-in
• Optional: try signing up from https://wordpress.com/log-in and/or https://wordpress.com/start/user after removing your Apple ID association with WordPress.

Fixes #37336

[matticbot]

Test live: https://calypso.live/?branch=update/siwa-change-login-return-url

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

App Entrypoints (~59 bytes added 📈 [gzipped])

name         parsed_size           gzip_size
entry-main         +65 B  (+0.0%)      +30 B  (+0.0%)
entry-login        +65 B  (+0.0%)      +29 B  (+0.0%)

Common code that is always downloaded and parsed every time the app is loaded, no matter which route is used.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[astralbodies]

Seems to work for me! I'm having some strange issues with Charles that's not getting me to a logged-in state though after it redirects.