feat: explicit hook configuration via addHook

[Fdawgs]

Supersedes #207.

This PR updates the addHook option to allow the hook type to be explicitly configured to be either onRequest or preParsing.
The hooks are limited to these two values so that developers cannot open themselves up to DoS attacks.
If the option is nullish or true it defaults to onRequest, which is its original functionality.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

Readme.md:83

• The addHook option description is duplicated. Please remove the redundant description.

* `addHook`: If `false`, this plugin will not register any hook automatically. Instead, it provides two decorations: `fastify.verifyBearerAuth` and `fastify.verifyBearerAuthFactory`. If `true` or nullish, it will default to `onRequest`. You can also specify `onRequest` or `preParsing` to register the respective hook

lib/errors.js:6

• The error message for FST_BEARER_AUTH_INVALID_HOOK is clear and helpful.

const FST_BEARER_AUTH_INVALID_HOOK = createError('FST_BEARER_AUTH_INVALID_HOOK', 'options.addHook must be either "onRequest" or "preParsing"')

[jsumners]

This text does not make it clear to me that it accepts the strings "onRequest" and "preParsing" to specify the hook to be used. It wasn't until I was reviewing the code changes that I understood the selection is meant to be provided here.

[Fdawgs]

@jsumners Hopefully a bit clearer in 341250b