Allow "unknown" values to be boomified

#### Support plan

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

#### Context

* *node version*: *
* *module version*: 9.1.4
* *environment* (e.g. node, browser, native): *
* *used with* (e.g. hapi application, another framework, standalone, ...): Typescript >= 4.4
* *any other relevant information*: *

#### What problem are you trying to solve?

Since typescript 4.4, the type of the `err` in a catch block will be "unknown".

```ts
try {
// stuff
} catch (err) {
throw boomify(err) // <=== typescript error "Argument of type "unknown" cannot be assigned to type: Error"
}
```

There are two solutions, both of which result in a lot of added code

- First:
```ts
try {
// stuff
} catch (err) {
throw err instanceof Error ? boomify(err) : err
}
```

- Second:
```ts
// we have to import this everywhere when it is needed
const asError = (err: unknown): Error => err instanceof Error ? err : Object.assign(new Error(err?.message ?? 'Unknown error'), err)

try {
// stuff
} catch (err) {
throw boomify(asError(err))
}
```

Since `boomify` is already an error wrapper utility, having to do either of these is anoying.

#### Do you have a new or modified API suggestion to solve the problem?

Could we allow the first argument of `boomify()` to be of type `unknown` ?

If you don't want to change the current signature, we could also make this optional:

```ts
boomify(err as unknown, { allowUnknown: true })
```