helmet contentSecurityPolicy directives depending on req.query

#### You have already researched for similar issues?
yes

#### What are you trying to achieve, or the steps to reproduce?
Need to set helmet contentSecurityPolicy directives depending on req.query
only if req.query.parent is set, and set directive to this value

```js
fastify
.register(helmet,
{
contentSecurityPolicy: {
directives: {
frameAncestors: [`${req.query.parent}.mydomain.com`, `domainB.com`],
}
}
}
```

#### Context

I don't know what syntax to use to set a dynamic directives value depending on `req.query`

This is a static value:
```
fastify.register(
helmet,
{
contentSecurityPolicy: {
directives: {
defaultSrc: ["'self'", 'default.com'],
...etc...
},
}
}
)
```

and this sets `res` value
```
contentSecurityPolicy: {
directives: {
defaultSrc: ["'self'"],
scriptSrc: [
function (req, res) {
// "res" here is actually "reply.raw" in fastify
res.scriptNonce = crypto.randomBytes(16).toString('hex')
}
],
```

Thanks