abstract the plugin to provide support of virtual file system

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the feature has not already been requested

### 🚀 Feature Proposal

I believe it is a good idea to provide an interface for file serving through different storage engine.
For example,
1. File System (which is currently)
2. MongoDB GridFS
3. S3 compatible interface

### Motivation

Manage of `Range` header is hard and maybe problematic.
Instead, we can provide a generic interface for the user to choose storage engine.
Then, we provide all the others function.

It enables the user to highly customize how to serve file based on their application logic and provide a proper storage engine.
Even file system serving can be customize in this case.

### Example

```js
import Fastify from 'fastify'
import FastifyStatic from '@fastify/static'
import FastifyMongoDB from '@fastify/mongodb'
import { MongoDBGridFS } from '@fastify/static/engine'

const fastify = Fastify()

fastify.register(FastifyMongoDB, {
// proper configuration here
// not to demo how to use mongodb
})
fastify.register(FastifyStatic, {
engine: MongoDBGridFS,
db: fastify.mongo.db,
bucketName: 'vfs'
})

// you can serve the file like currently
// but it is backed by other engine
```

No files or tests are named. Start by reading the plugin's current file-serving and Range-header handling, then examine how a generic storage-engine boundary could support the existing filesystem path and the proposed MongoDB GridFS example. Done would require an agreed design that preserves current serving behavior while enabling alternate storage engines.

Written by the indexing model from the issue text.