chore: refactor CLI downloading logic

I moved and modified the logic from CliManager.download to a separate
http client based on okhttp and retrofit. The refactor will allow us
to easily add new steps in the main download method, and also to easily
download new resources. Long term we could also re-use the okhttp client
to avoid setting twice the same boilerplate (proxy which is missing from CLIManager,
hostname verification and other tls settings) between cli downloader and the rest client

Author: fioan89