feat: multi account user data support

[grumbach]

Before:

  ~/.local/share/autonomi/client/
  ├── user_data/
  │   ├── registers/
  │   └── scratchpads/
  ├── register_signing_key
  └── scratchpad_signing_key

After:

  ~/.local/share/autonomi/client/
  ├── 0x742d...aFb1/
  │   ├── user_data/
  │   │   ├── registers/
  │   │   └── scratchpads/
  │   ├── register_signing_key
  │   └── scratchpad_signing_key
  │
  └── 0x8626...1199/
      ├── user_data/
      │   ├── registers/
      │   └── scratchpads/
      ├── register_signing_key
      └── scratchpad_signing_key