Flatten glob "assets" to one output folder

# 🚀 Feature request

```
- [ ] new
- [x] build
- [x] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [ ] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc
```

### Description
I am currently working with nested assets folder per entity map. Currently, I am using the following assets export settings:

```json
{
"glob": "**/assets/**/*",
"input": "src/app",
"output": "/assets",
"ignore": ["**/*.ts"]
}
```

Imagine having the following file structure:

```
src
├── app
| ├── module-folder
| | ├── assets
| | │ ├── images
| | │ │ └── image1.png
| ├──other-module-folder
| | ├── assets
| | │ ├── images
| | │ │ └── image2.png
```

### What I get

This would result in the following `dist/` file structure:

```
dir
├── assets
| ├── module-folder
| | ├── assets
| | │ ├── images
| | │ │ └── image1.png
| ├──other-module-folder
| | ├── assets
| | │ ├── images
| | │ │ └── image2.png
```

### What I want to have

```
dir
├── assets
| ├── image1.png
| └── image2.png
```

### What I have tried

I have tried to use a custom script to move the assets files manually in a flatten way to the dir folder.

```sh
for file in ./src/app/**/assets/**/*.*; do cp "$file" "./dist/assets/$(basename -- "$file")" ;done
```

This, however, does not work in combination with `ng serve` as the files are in-memory, and also does not work with the `ng build --watch` as I am unsure how to activate script after it has been built.

---

I would love to get some insight into the best approach. Maybe this is simply using an already existing flow. From my point of view, this should be included as functionality, as the nested folder structure strategy seems to be one of the more common approaches.

Inizia dalla configurazione dell’esportazione degli asset e confronta il modo in cui ng serve, ng build --watch e una build normale elaborano il glob mostrato. La modifica è completata quando gli asset annidati possono essere emessi direttamente sotto dist/assets senza le cartelle intermedie module e assets, rimanendo disponibili anche per il workflow di serve in memoria.

Scritto dal modello di indicizzazione a partire dal testo della issue.