Add new command which allows downloading new Shlink versions

[acelaya]

This tool currently allows to set-up a new Shlink version, importing configuration from the previous one.

However, users are still responsible of manually downloading the new dist file. It would be good to provide some tooling capable of doing that.

Considerations:

• It should limit how many versions you can "jump" in one go. For major versions, it would require you to be in the last minor version of the previous major.
• It should create a backup of the previous version, ensuring a rollback is possible if needed.
• It should ideally allow to rollback too, taking care of running database migrations "down".