Allow disabling the update checker

[raxod502]

Is your feature request related to a problem? Please describe.
Objection appears to check for updates automatically. This is not helpful to me, as I install software through a package manager on a specific schedule except in emergencies. I prefer not to have any of my software making unnecessary network calls.

Describe the solution you'd like
An environment variable, command-line option, or other straightforward way that I can disable update checking when packaging Objection for my system's package manager.

Describe alternatives you've considered
I believe the alternative is I need to patch out Python code when packaging.

Additional context
See also #744, which makes the problem worse.

[hiSandog]

A package-friendly setting should guarantee that startup performs no update-network request, not merely hide the notification. It would be useful to support a persisted config key plus an environment/CLI override, document precedence, and make --offline imply the same behavior. A test with a blocked/mock network can assert that the checker is never instantiated when disabled.