Custom error message for each validation step.

#### Context

* *node version*: 16.4.0
* *module version*: 17.5.0
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): nestjs
* *any other relevant information*:

#### How can we help?

Is there a better way to set custom error messages for each validation? Similar to Yup, is it possible to add custom messages in the validation function itself `.string().required('Custom required message')`. I can see the point to add all custom messages for the schema in a separate property where all messages can be accessed, but having this feature could be easy for understanding validations and their error messages.

```js
// Current implementation

const schema = Joi.object({
username: Joi.string()
.min(3)
.max(30)
.required()
.messages({
"string.min": "Must have at least 3 characters",
"string.min": "Must have max 30 characters",
"string.required": "This is required"
})
};

// Is something like this possible?

const schema = Joi.object({
username: Joi.string()
.min(3, (len) => `Must have at least ${len} characters.`))
.max(30, (len) => `Must have max ${len} characters.`))
.required("This is required")
};
```

The second option looks more readable and clear to me. Is there an option to achieve this?