FR: Allow to hide UI boxes and overflow their content in containing box (see #932)

See #932 for context. This is suggestion 3.

The configuration structure is naturally inherited from the `values.yaml` which is sometimes a bit too deeply hierarchical whereas forms are usually more linear. The `Custom` box, below, for example, doesn’t add much value to the interface.

It would be useful to allow some hierarchical boxes to be hidden, with their content "overflowing" into the containing box.
This should also be controlled in `values.schema.json`:

Proposition 1:
```json
{
"type": "object",
...
"x-onyxia": {
"showBox": false,
"hideBox": true # Alternatively
},
}
```
Proposition 2 (this would also allow more rendering customization options in the future)
```json
{
"type": "object",
...
"render": "none" # or "dissolved", alternatively
"x-onyxia": {
...
}
}
```

**Note: It would be very useful to be able to hide the topmost dependency subchart section.**

Customizing an already onyxia-aware chart A can be done by defining a new chart B with a single dependecy to A and then redefining the `values.schema.json`. However all values defined in A are now nested in a topmost object and all associated configuration is cramed into a single tab in the UI. Dissolving that topmost tab should allow to get back the presentation that we would get if we directly used the A chart.

*edit: this last bit is actually less relevant as the latest version of Onyxia already dissolves configuration from direct subcharts. However I am not sure it is the case for longer dependency chains (sections in subsubcharts for instance).*