Access global object property from relative property key

#### Support plan

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

#### Context

* *node version*: V14.latest
* *module version*: 17.latest
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): with express, but the issue exists as standalone
* *any other relevant information*:

#### How can we help?
I'm trying to set a default value of a property based off of a global context passed through at `validate` and an existing property of the object being validated. I just can't seem to use the value of the property of the object as a key for the global context.

```js
const context = {
letters: {
a: 1,
b: 2,
c: 3,
},
};

const s = Joi.object().keys({
letter: Joi.string().valid(...['a', 'b', 'c']),
num: Joi.number().default(Joi.expression('{$letters}.{letter}'))
})

const test = {
letter: 'a',
}

console.log(s.validate(test, { context }));
```

Simple example above. I have an object called `letters` that I place into the context. The schema will look for `letter`, then try to set a default value for `num` using the global letters as the object to pull from and the letter passed in as the key. The closest I can get is `{ value: { letter: 'a', num: '[object Object].a' } }`

The report provides no repository file or test path. Start by reproducing the Joi.expression default in the supplied Node example and inspect the Joi.expression and validate entry points. Done means a relative property key selects the matching value from the global letters context, producing num: 1 for letter: 'a'.

Written by the indexing model from the issue text.