Improve otp subcommand

[robinkrahl]

Similar to the PWS refactoring discussed in #139, we should also update the OTP commands:

• Introduce otp add command
• Introduce otp update command
• Remove or deprecate deprecate otp set command
• Support reading the secret from stdin and prompting it (Refactor otp set command #167)

[robinkrahl]

I just realized that we cannot implement this yet because we can’t query the TOTP time window value, see Nitrokey/libnitrokey#160.