feat: Allow users to choose the behaviour of the update.

[GabuTheDev]

Currently, tosu checks for updates (if the toggle is on), downloads the update and reboots automatically.

We should let the user choose the behaviour. To list a few:

• automatically restart after downloading (current)
• require user restart to apply update manually (meaning it will install, but not restart automatically)
• snooze the update (until next start)

This means the current setting Auto Update Client should turn into two: Check for updates and Update behaviour (or update method, whatever wording works)

[KotRikD]

--update=false

[GabuTheDev]

--update=false

if the flag is disabled, it checks for updates and announces there's a new version without actually updating.
I was suggesting an option where the update still happens, but doesn't restart the current session. Two different things.