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.

Comece pela configuração de exportação de assets e compare como ng serve, ng build --watch e um build normal processam o glob mostrado. A alteração estará concluída quando assets aninhados puderem ser emitidos diretamente em dist/assets sem as pastas intermediárias module e assets, permanecendo disponíveis para o fluxo de trabalho de serve em memória.

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