Allow easy switching of SDK versions from `dotnetup`

`dotnetup` should support easily switching between .NET SDKs on a per-session basis:

```shell
> dotnetup use 8.0.403
> dotnet --version
8.0.403
```

This feature enables easy side-by-side testing of SDKs in a way that doesn't permanently impact the users' workspace - for example for side-by-side testing of an experience like `dotnet watch` across diferent SDK releases.

There should also be a way to _persist_ that change permanently to a global.json:

```shell
> dotnetup use 8.0.403
> dotnet --version
8.0.403
> dotnetup use 8.0.403 --save
Wrote SDK version 8.0.403 to ./global.json
```