Introspect OIDC token to fix automatic Thundermail CalDAV connection

[davinotdavid]

What changed?

During the automatic Thundermail CalDAV connection, instead of using Subscriber.email for the auth, we are introspecting the OIDC token and using a fallback of preferred_username -> username -> Subscriber.email.

Why?

We shouldn't rely on Subscriber.email to attempt to connect with the Thundermail CalDAV. The credentials should be derived / introspected from the OIDC token instead.

How to test

This might be a bit troublesome to test but you'd need to run both thunderbird-accounts and appointment in your local dev environment and change the .env vars from appointment to point to thunderbird-accounts ports. Also don't forget to change the AUTH_SCHEME to oidc.

1. Navigate to `http://localhost:8090
2. Sign up with admin@example.org / admin (if this works, you have successfully setup both projects together)
3. In the FTUE, click on "Connect Thundermail Calendar" and click Continue
4. Check that there shouldn't be any errors

Applicable Issues

Closes #1757
Related with #1689

[devmount]

Code lgtm, tests are passing (in CI and locally for me too), CalDAV still works as expected when navigating the frontend.

I wasn't able to go through the tests steps though, since I don't have a working tb-accounts instance running locally 😇