Implement a non-interactive keystore import feature

[remyroy]

Is your feature request related to a problem? Please describe. As a tool developer on top of various clients and other tools, it would be nice to be able to non-interactivly import password-protected keystore files.

Describe the solution you'd like Add a secure way to pass the keystore password to the deposits import sub-command and a non-interactive switch/flag for automation.

Describe alternatives you've considered We are currently automating feeding the password to stdin when calling deposits import but that interface might be unstable. We are thinking about using the keymanager APIs when it will be ready across the board.