Is it possible to use external parameters in an extension?

#### Context

* *node version*: 14.x (aws lambda function)
* *module version*: 17.4.2
* *environment* (e.g. node, browser, native): node in aws lambda
* *used with* (e.g. hapi application, another framework, standalone, ...): standalone
* *any other relevant information*:

#### How can we help?

I'm curious if this is something I can do in JOI, but basically I have a user that is sending a request to an API to store a specific type of metric data. Certain metrics however are user role restricted, so I want to be able to use JOI to 1) validate the user is trying to store a valid metric and 2) validate that the user has the proper role to store that metric.

I've tried using context and passing that into the extended validator but I've had no luck in getting that to work, this is ideally what I'd like to be able to do:

From main function:
```node
let userMetric = {id: "metric1", label: "Metric 1"};
let userRoles = ["metric2access"];
const { error, val } = schema.validate({ metric: userMetric }, {context: {userRoles: userRoles }});
```

Schema:
```node
const Joi = require('./customjoi')

let validMetricIds = ["metric1", "metric2"]

const schema = Joi.object().keys({
metric: Joi.object({
id: Joi.string().roleCheck({userRoles: '$userRoles'}).valid(...validMetricIds)
})
})
```

customjoi.js
```node
module.exports = Joi.extend((joi) => ({
type: "string",
base: joi.string(),
messages: {
"string.roleCheck": "User does not have access to {{#label}}"
},
rules: {
roleCheck: {
params: {
options: Joi.object({
userRoles: Joi.any()
})
},
validate(params, value, helpers) {
let metricsRequiredRoles = {"metric1": ["metric1access"], "metric2": ["metric2access"]};
if (Object.keys(metricsRequiredRoles).includes(value)) {
requiredRole = metricsRequiredRoles[value];
if (params.options.userRoles.includes(requiredRole)) {
return value;
} else {
return helpers.error("string.roleCheck")
}
}

return value;
}
}
}
}))
```

Is this type of thing possible with JOI? Is there another way to do it that's not like the way I wrote here? I need to be able to reference the user's current roles while validating.