Client cleanups

[conradoplg]

Just to keep a record of things we want, we can split into issues later

• Improve config file management #379
• Add a name/description field to groups so the user can more easily recognize which group is which #378
• Support multiple sessions (allow selecting which session to use) #376
• Allow editing/removing entries from the config file #377
• Refactor/organize the whole project, e.g. move participant and coordinator to inside frost-client. Possibly remove their old CLI interface? #380
• Create a type for public and private communication keys instead of using Vec<u8> all around