[Enhancement] Allow ⪆ `31.0.10` to act as a CalDAV client.

[RokeJulianLockhart]

What Problems This Relates To

I've two relevant desires:

1. When I add NextCloud as a source in an external synchronisation service — utilise davx5-ose as an applicable example — I want external CalDAV sources, added via Tasks' GUI, to be available.

2. I want to be able to access external CalDAV sources in Tasks' GUI.

One might consider davx5-ose to solve this. However:

1. It requires an external synchronisation service (of which NC and SyncThing are examples) to synchronise its preferences. However, even that basic form of synchronisation requires periodic manual exportation and importation, which is brittle. NextCloud does not suffer from this.

2. It's not available outside of AOSP.

3. Utilising an external client renders NextCloud a purely backend service, which likely isn't what NC desires for its Tasks application.

Despite this, I don’t see any way to add an external CalDAV source to enumerate tasks from, like one can for iCal sources in Calendar. (Although Calendar accepts URIs to CalDAV sources, that appears to be erroneous.)

Desired Solutions

Implement a two-way equivalent to Calendar's one-way iCal subscription GUI, but for CalDAv sources.

Considered Alternatives

Utilise an external client, of which tasks/tasks is an example, with an external synchronisation service, of which davx5-ose is the best example. However, consider its (aforestated) comparative disadvantage.

Additional Context

Originally discussed at help.nextcloud.com/t/234881/4.