feat: quickget could store ISOs in a cache directory and link there, rather than redownloading, quickemu could use that cache path

[JonTheNiceGuy]

I confirm this feature has not been previously requested

• I have searched the issues and this feature has not previously been requested

Is your feature request related to a problem? Please describe.
As a new user of quickget, I ran quickget windows 11, started the vm, finished my changes, then ran rm -Rf windows-11*. I then needed to try it again, so I ran quickget windows 11. It downloaded the ISO again. After several loops of this, I was blocked by Microsoft from downloading this ISO.

Describe the solution you'd like
A new option for quickget which puts the downloaded ISO in a known path (e.g. $HOME/.cache/quickemu) and then sets the config file with iso="$HOME/.cache/quickemu/some-iso-file.iso".

Describe alternatives you've considered
A config file (e.g. $HOME/.config/quickemu/settings) which, if a value there (e.g. iso_cache="$HOME/.cache/quickemu") defines the cache so when the machine config file lists iso="cache/some-iso-file.iso" quickemu will recognise the cache prefix as the defined cache directory, and rewrite the path accordingly.

I have also noticed that you can do quickget --download windows 11 and then quickget --create-config windows 11 but that moves the ISO into the stored path (a hard link might also be better?).

Additional context
Something akin to how Vagrant Boxes are stored would be great :)