operator generate-root -decode: allow passing encoded token via stdin

[davidducros]

Is your feature request related to a problem? Please describe.
In our environment processes and their arguments are logged. Passing both the encoded token and the otp as command line arguments would allow anyone with access to these logs to decode our Vault root token.

Describe the solution you'd like
The solution proposed in #12881 allows passing the encoded token via stdin rather than a command line argument. Passing the otp via stdin was another option but it's used across a few commands.

[ccapurso]

Hi @davidducros, thank you for your submission! I have provided feedback from my review of the referenced pull request.