Thin online installer

### Checklist

- [x] I added a descriptive title
- [x] I searched open requests and couldn't find a duplicate

### What is the idea?

anaconda used to have these "shared environment files". The idea was to make it is easy to install pretty complicated environments quickly.

I would like to recreate something similar for users that do not have conda installed at all.

This would allow:
1. Providing users a "small" download.
2. That download pulls in a much more elaborate installer

### Why is this needed?

Running

```
conda env update --file SOME_SPEC_FILE
```

after the creation of the base environment

### What should happen?

Downloading optimized packaged based on cpu capabilities and potentially GPU.

### Additional Context

could be done with https://github.com/conda/constructor/issues/985