[Suggestion] Add a distrobox save and load command

**Is your feature request related to a problem? Please describe.**
I'm always frustrated when I need to export a container to transfer it on another machine or trough a new host installation and I never remember the commands needed (because I rarely do this) so every time I have to go and look it up when it seems just trivial to implement.
This is good advertisement and the first thing I name when I talk about distrobox, unfortunately though, it's needlessly annoying to do such a simple operation. Distrobox should have a feature to save a container onto a file.

**Describe the solution you'd like**
Add a distrobox-save command from the host sistem or `distrobox-export --container` from within the container itself.
Of course a `distrobox-import` or `distrobox-load` would also need to be implemented.

**Describe alternatives you've considered**
I've considered making another wrapper on top of distrobox or using shell aliases, but then other people would also need to have this additional software in order to use this and talking about this as a "distrobox feature" would be dishonest at best.

**Additional context**
I could work on this myself as it seems to be a fairly trivial task and it's documented enough.
https://github.com/89luca89/distrobox/blob/main/docs/useful_tips.md#container-save-and-restore