Client cleanups

[conradoplg]

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

• Improve config file management - currently if you want two instances of frost-client concurrently they might corrupt the config gile. See how cargo does it https://github.com/rust-lang/cargo/blob/eaee77dc1584be45949b75e4c4c9a841605e3a4b/src/cargo/util/context/mod.rs#L2328
• Add a name/description field to groups so the user can more easily recognize which group is which
• Support multiple sessions (allow selecting which session to use)
• Allow editing/removing entries from the config file
• Refactor/organize the whole project, e.g. move participant and coordinator to inside frost-client. Possibly remove their old CLI interface?