[massa-client] handling multiple wallet files

Currently, we cannot use two different wallet files in `massa-client/wallets/`, e.g. yaml files created with Massa Station with different passwords: the client will prompt for a password and fail.

Improvements:
- `wallet_info` should not ask for password as it does not sign anything
- commands that sign something should ask the password of the corresponding address (designated in the command) if a corresponding file is found in `wallets/` otherwise fail
- `wallet_get_secret_key` could be used with only one address instead of several, to ask only one password
- match the vocabulary with the usage: when we read comments like `wallet_add_secret_keys ...: add a list of secret keys to the wallet`, we understand that a wallet can contain several addresses so a wallet file should be able to contain several addresses with the same password, and the client should integrate them to the file when using commands like `wallet_add_secret_keys` and `wallet_generate_secret_key` (e.g. with the wallet nickname as parameter), while instead curently they create a new wallet in `wallets/`