Client: avoid showing/using secret key and wallet password if we don't need to

[sebastien-forestier]

Currently, when launching the client, it asks for the wallet password whereas there are plenty of use cases where you don't need the wallet (get_status) or the password (wallet_info to see my balance).

We should:

• not ask the wallet password until we want to sign something
• not show the secret key unless we really need it (for instance we could ask it through a get_secret_key(address)), and do not show it in wallet_info,
• not show the public key either (use get_public_key(address) if you need it) as it can be confused with address
• node_add_staking_secret_keys secret key handling should be done automatically, and DO NOT MAKE THE USER COPY/PASTE THE SECRET KEY, for instance with node_start_staking(address).