Validation - only allow SINGLE LAYER of nesting for a property whose value is an object

#### Support plan

* *is this issue currently blocking your project?* (yes/no):
* NO
* *is this issue affecting a production system?* (yes/no):
* NO

#### Context

* *node version*: 14
* *module version*: 17
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): expressjs
* *any other relevant information*: nil

#### What problem are you trying to solve?
Is there a way to only allow a SINGLE layer for a property which is an object? meaning no nesting... lets take the following example:

```javascript
{
"hello": {
"foo": 1,
"bar": 2
},
"world": 3
}
```

the property `hello` is an object of `{"foo": 1, "bar": 2}`.

This should pass validation.

If we were to add an object that is further nested, then it will fail, for example:
```javascript
{
"hello": {
"foo": 1,
"bar": {
"good": 4,
"bye": 5
}
},
"world": 3
}
```
In essense, how do I ensure only a single layer and no nesting for a selected property, which has an object as a value?

#### Do you have a new or modified API suggestion to solve the problem?
No

The issue names no source file, test, or entry point. Start by checking Joi's existing object-validation API against the two example objects; done would mean a schema accepts the one-level object and rejects the nested object.

Written by the indexing model from the issue text.