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.

Start with the asset export configuration and compare how ng serve, ng build --watch, and a regular build process the shown glob. The change is done when nested assets can be emitted directly under dist/assets without the intermediate module and assets folders, while remaining available to the in-memory serve workflow.

Written by the indexing model from the issue text.