Allow more secure chia keys add options

[ericaltendorf]

Currently it appears the only way to add new key by mnemonic is by putting it in a file:

  -f, --filename TEXT  The filename containing the secret key mnemonic to add
  -l, --label TEXT     Enter the label for the key
  -h, --help           Show this message and exit.

Persisting in a (plaintext) file, even one you intend to delete immediately, opens the door for accidental or malicious exposure of mnemonics to other users or services on the machine. (Imagine if you had a backup service running, say.)

An alternative, entering mnemonics via a command line argument, also seems insecure as most shells will save the information in your shell history.

The most secure way to enter a plaintext mnemonic seems to be to interactively prompt the user to enter the mnemonic, so the text goes straight to the process. Probably this should also not echo the characters back to the screen, since those will get saved in terminal or terminal multiplexer scroll buffers. This may end up as a web password style enter twice and compare type thing.