feat: add a "--user-agent" parameter to `wget` calls

Seems like it might be helpful (and polite) to differentiate our visits from the noise.

I think if you would set a custom user-agent string, instead of the default wget that would already help to identify your tool better at reverse proxy level / fail2ban etc. By simply using --user-agent=deb-get/<version> parameter in this case.

Originally posted by @melroy89 in #1699