Spaces: API attribute "sync-local-parent" to keep locally synced Spaces in a common parent folder (Desktop)

## Is your feature request related to a problem? Please describe.

@hodyroff brought up, regular users expect to sync [all Spaces to local disk by default](https://github.com/owncloud/ocis/issues/2793), and Spaces should be placed in a local "`Spaces`" parent folder.

```
.
├── Home
├── Shares
│   ├── FromAlice
│   └── FromBob
└── Spaces
├── Space1
├── Space2
└── Space3
```

## Describe the solution you'd like

Flexible solution would be, Spaces API could get an additional attribute "sync-local-parent" to control the default behaviour of the desktop sync client.

In the first iteration, this attribute could be hardcoded – no backend logic needed.

In the future, we could also support scenarios with all Spaces in account root:

```
.
├── Home
├── Shares
│   ├── FromAlice
│   └── FromBob
├── Space1
├── Space2
└── Space3
```

…or use different terminology:

```
.
├── Home
├── Shares
│   ├── FromAlice
│   └── FromBob
└── Project
├── Project1
├── Project2
└── Project3
```

## Describe alternatives you've considered

- Not place all Spaces in a common folder "Spaces"
- Hardcode in desktop sync client to place all Spaces in a common folder "Spaces"

Start by tracing the Spaces API response and the desktop sync client's handling of synced Spaces. Define the first-iteration contract for the hardcoded "sync-local-parent" attribute, then verify that Spaces are placed under a common local "Spaces" folder without requiring backend logic.

Written by the indexing model from the issue text.