Add support for extension `recommendations` and `unwantedRecommendations` in devcontainer.json `customizations:vscode:settings:extensions`

Currently extension `recommendations` and `unwantedRecommendations` can only be set in _.vscode/extensions.json_ file.

Adding support for setting those in _devcontainer.json_ under `customizations:vscode:settings:extensions` key would allow avoiding the need for creating (and adding to **git**) the _.vscode/extensions.json_ file.

Something like:
```json
{
"customizations": {
"vscode": {
"settings": {
"extensions": {
"recommendations": [],
"unwantedRecommendations": []
}
}
}
}
}
```

This would be the "equivalent" for `customizations:vscode:settings:launch` (that "mimics" _.vscode/launch.json_) and for `customizations:vscode:settings:tasks` (that "mimics" _.vscode/tasks.json_).