[FEATURE REQUEST] Rustup update support

### Please confirm these before moving forward.

- [x] I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
- [x] This proposal is a completely new feature. If you want to suggest an improvement or an enhancement, please use [this template](https://github.com/Devolutions/UniGetUI/issues/new?labels=enhancement&projects=&template=enhancement-improvement.yml&title=%5BENHANCEMENT%5D+%28Enter+your+description+here%29).

### Describe the new feature

I would love to see rustup updates in the updates list.

### Describe how this new feature could help users

Currently users are not aware their rustup version is outdated.

Only if they regularly run the `rustup update` command, they're getting updates.
But be honest: no one really does this. Or want to do this.

Since UniGetUI already supports Cargo packages, why not allow to update rustup itself as well?
Of course rustup is not a package manager, but a cargo version manager, but still