allow "default virtual" for new shares

## Feature request

**Which Nextcloud Version are you currently using:** 30

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

Sort of. When I share a new folder to the client, that share has lots of files, and the intent is for that share to be part of the virtual share, the desktop client always defaults to downloading everything. Because the client app is not fully async, getting it to _stop_ syncing and allow us to set it as "save space" in virtual-file mode, occasionally we have to kill the app.

The async and hanging app thing is not for this issue, they are discussed elsewhere.

**Describe the solution you'd like**

What I'd really like to see is a checkbox under the VFS tab akin to

```
[ ] Use VFS by default for new shares
```

This is _much_ faster for all shares, and is more easily changed on a case-by-case basis when some shares are rather large.

**Describe alternatives you've considered**

While it may be possible to hand-edit the underlying config files, I don't consider this a good solution.