Added a way to auto update av definitions

[pballandras]

Hi cvdupdate team,

We're currently using your lib with our clamav setup and we found that it was inconvenient for us to run crons, so we implemented an optional updater that can run beside the server. We didn't change the current behaviour as everything is optional so current users won't see a difference if they don't want the feature.

What got added

• auto_updater.py: Enables starting definitions updates after a certain interval. This is disabled if the interval is 0.
• A call to the auto_updater.start(interval: int) method in the cvd serve call. This is parametrized with --update-interval-seconds, a INT option in click which by default disables updates.

Thanks for your time!

PS: If you want me to edit the README, I'll be glad to do so

[micahsnyder]

Thanks for submitting the new feature @pballandras. I'll review it as soon as I'm able. Thank you for your patience.

[micahsnyder]

This is really cool. I don't see anything concerning.
I appreciate that you've made it default-off by setting the default interval to 0.

Thanks again!