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.

從資產匯出設定開始，比較 ng serve、ng build --watch 和一般建置處理所示 glob 的方式。當巢狀資產可以直接輸出到 dist/assets 底下，不需要中間的 module 和 assets 資料夾，同時仍可用於記憶體中的 serve 工作流程時，此變更即完成。

由索引模型根據 Issue 內容生成。