Implement "Online Accounts" Portal

## Problem

Flatpacked apps using EDS are not able to retrieve credentials from the host system.

## Proposal

What we really want for this is a [Flatpak Portal](https://docs.flatpak.org/en/latest/desktop-integration.html#portals). Here's the corresponding discussion from Slack:

You need an authorization flow that would be like this:
- Have an "Add account…" on the app (that sends some settings to the portal like supported auth type (ex: password), account types (ex: imap+smtp or pop3+smtp for password)
- Then the portal process the querry,
- if you already have an account configured that is not yet allowed, show a dialog with a list of available accounts and a "Add new account…", clicking on an account would return it to the app, clicking on the "Add new account…" would go next case
- if you don't have an account configured (or you clicked "Add new account…"), then the online account plug is opened at the "Add account" page

So basically, that's not just a credential portal, we don't want to mimic libsecret on D-Bus which isn't exposed for reasons - because we need a way to tell if the credentials requesting app is authorized. Otherwise a malicious app could request any credentials and/or spoof another app's id to do so. So a simple DBus handing over whatever credentials are requested is a serious security hole. That's why it's a portal, you always know the origin of the request that way.

We should be able to start out with a minimal proof of concept: A portal supporting the full flow for CalDAV accounts only (since this is essentially only three properties uri, username and password) - and then build from there for IMAP/SMTP, OAuth, ... Also remember that for OAuth, each app will need to provide a different Client Secret, only elementary official apps can use the elementary one.

## Related Issues

- https://github.com/elementary/switchboard-plug-onlineaccounts/issues/209
- https://github.com/elementary/tasks/issues/263
- https://github.com/elementary/mail/issues/591

Start with the linked Flatpak Portal documentation and the related Online Accounts issues, then map the proposed authorization flow for a CalDAV-only proof of concept. Done means an app can request an authorized account and receive its URI, username, and password without exposing arbitrary host credentials.

Written by the indexing model from the issue text.