Refactoring client

[jpraynaud]

Issue

We need to do some complementary refactoring on the client

To do

• Enhance json output download sub-command
• Check if download_directory is writable before downloading certificate chain
• Progress bar during download UI (indicatif only when not using --json option)?
• Check if enough available space to download and unpack snapshot (maybe with du-dust if it's possible)

Later

• Progress bar during download (with --json option, TBD)?
• Optimize certificate verifier and genesis verifier usage in commands?
• Reactivate client cache implementation for immutable digester?