feat: extends send result to provide ability of custom handling

[climba03003]

I would like to extends the result to provide ability of custom handling. Related to fastify/fastify-static#454

I don't like the hooks or event approach because it is more explicit and requires the users to do more fundamental works.
For example,

1. providing the directory listing no longer requires the users to handling the trailing slash case.

Benefits of the current approach,

1. Do not needs to handle the async-await, callback or sync function.
2. Ensure consistence result.

Drawback of the current approach,

1. We create a stream no matter the users will be use it or not.

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

[gurgunday]

I also prefer this

We create a stream no matter the users will be use it or not.

I mean the user should expect to have a stream after a successful call to send in my opinion

[climba03003]

I do see some case that will not be able to cover.
Currently, @fastify/static is depending on the case before the trailing slash handling.
https://github.com/fastify/fastify-static/blob/next/test/dir-list.test.js#L146-L172
Since, 403 is not unique status code fired inside sendRedirect.
It is hard to support the directory listing in fastify.

For example,

send/lib/send.js

Lines 592 to 594 in efbaed1

	if (hasTrailingSlash(options.path)) {
	return sendError(403)
	}

Edit: Nevermind, I manage to handle the problem on @fastify/static.

[mcollina]

lgtm