Support customization of the authentication failed status code

[pqnet]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

I suggest to introduce the ability to customise the status code for failed authentication requests

Motivation

A follow-up issue to #164. To support using Proxy-Authenticate in a manner that is compliant to the http standard it is necessary to use the 407 status code in the error response. See

https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Proxy-Authenticate

Example

options = {
    authenticate: { realm: "My Realm", header: "Proxy-Authorization", failureStatusCode: 407 }
}

Originally posted by @pqnet in #164

[pqnet]

Looking at the code in https://github.com/fastify/fastify-basic-auth/blob/main/index.js#L6-L10 it seems to me that the 401 status code is kind of hard coded into the error, which is created once and re-used.

Consistently with the other changes we should let the status code be customisable as well.