extension object fails validation when used twice

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

#### Context

* *node version*: 14.x
* *module version with issue*: 17.3.x
* *last module version without issue*: 16.x
* *environment* (e.g. node, browser, native): all
* *used with* (e.g. hapi application, another framework, standalone, ...): standalone
* *any other relevant information*: input validation issue

#### What are you trying to achieve or the steps to reproduce?

Trying to pass an extension object instance (rather than a generator function) to Joi twice results in a validation error [due to a modification made by Joi](https://github.com/sideway/joi/blob/9cffab90039c313ca62d473d01993b492dd4d007/lib/extend.js#L111).

```js
import joi from 'https://cdn.skypack.dev/joi@~17.3.0';

const extension = {
type: 'foo',
base: joi.string(),
rules: {
bar: {
args: [{
name: 'baz',
assert: joi.string()
}],
validate(value, helpers, args, rule){
return { value }
}
}
}
}

// assume this is in the first library that uses Joi
const Joi = joi.extend(extension)

// assume this is another library or another direct usage. this call will produce an error
const Joi2 = joi.extend(extension)
```

#### What was the result you got?
Validation error.

#### What result did you expect?
For extension objects to be reusable without errors. Extend seems as if it should not have side effects on the extension objects passed.