[FEATURE REQUEST] central storage for downloaded update files

### 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

It would be a great feature to define a central storage where the downloaded update files are stored.
If you update more than one computer, all computer could use that storage an skip the download of an already downloaded installation file.

### Describe how this new feature could help users

Could save a lot of downloads a speedup the installation/updates.

The issue does not identify files, tests, or entry points. First clarify the central storage location, how downloaded files are identified and shared across computers, and which package managers are covered; done means updates can reuse an installation file from that storage instead of downloading it again.

Written by the indexing model from the issue text.