Auto-update index periodically

[mosteo]

Default every 24h. The user is asked the first time.

Fixes #1348

[Fabien-Chouteau]

I see why you want to do this, we regularly have questions from people that never update the index. However I am a but worried about the consequences :)

What happens if there's no internet connection?

[mosteo]

Actually I thought that you also wanted it :) given the discussion in #1348.

Yup, I forgot to test what happens in the offline case. We should fail the update gracefully with a warning and continue as before. We probably don't want to try again immediately but back-off for the configured period in any case. I'll do a bit more testing.

[rowan-walshe]

I like the change. I was once on of those that had to ask why alr couldn't find a newer version of a crate that I could see in the index on GitHub.

One comment I have is that while alr update --online can be used to force update the index(es) before attempting crate updates, it feels like it should still call Auto_Update_Index if --online isn't set

[mosteo]

Updated to ensure that an automatic update failure causes the regular backoff, and is handled as a warning.

Also added a test for it. I didn't find a simple solution to make the test locally, so that test uses an online repo I created to that effect. There may be solutions around that by using a local git server, but that seemed too complex just to avoid going online, given that that's not a crucial situation for our testing.