Retrieve list of valid values given a specific key from a list of keys in a nested object

#### Context

* *node version*: v16.14.2
* *module version*: 17.6.0
* *environment* (e.g. node, browser, native): Node
* *used with* (e.g. hapi application, another framework, standalone, ...): Standalone

#### How can we help?

Given the following schema:

```js
schema = Joi.object({
foo: Joi.string().valid('Lorem', 'Ipsom'),
bar: Joi.when('foo',[
{
is: 'Lorem',
then: Joi.string().valid(
'dolor',
'sit',
'amet'
)
},
{
is: 'Ipsom',
then: Joi.string().valid(
'Aut',
'atque',
'consectetur'
)
}
]
baz: Joi.when('bar',[
{
is: [
'dolor',
'sit',
],
then: Joi.string().valid(
'Eos',
'voluptas',
'fugiat',
)
}
{
is: 'amet',
then: Joi.string().valid(
'aut',
'accusamus',
'laboriosam',
)
}
{
is: [
'Aut',
'atque',
'consectetur'
],
then: Joi.string().valid(
'distinctio',
'dolores',
'asperiores',
)
}
]
})
```
I want to be able to given a string key, e.g 'dolor', I can retrieve the list of possible values easily. I can do this by adding an ID to the "then" statement, but this way I have to build a group reference and I cannot directly use my key to retrieve the list of possible values, I was expecting to do something like:

```js
const validValuesforBaz = this.schema.extract(`foo.${foo.Lorem}.bar.${bar.dolor}.baz`).describe()['allow'];
```

Is there a way I can use my object keys to retrieve the list of possible values? Do I need to change my schema?

Start with the schema.extract(...) and describe()['allow'] entry points shown in the example, alongside the Joi.when conditional branches and their then schemas. Determine whether a string key can address nested conditional values without IDs or whether the schema must change; done means the supported lookup returns the expected valid values for baz.

Written by the indexing model from the issue text.