Drop github.com/urfave/cli/v2 dependency

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when we're using a lot of similar libraries, especially if we're using two different ones in the same project. S3 gateway itself doesn't use github.com/urfave/cli/v2, but authmate for whatever reason does.

Describe the solution you'd like

Unify with S3 (and even REST) gateway, drop github.com/urfave/cli/v2 dependency.

Describe alternatives you've considered

Leave as is.