airdrop onboarding flow for Namada CLI

[gavinly]

for onboarding airdrop recipients, we'll want a CLI flow that walks the user, step by step, through the process of generating the keypair for receiving their airdrop with these key elements:

1. give the user the option to use a Ledger hardware wallet or to write down their private key info
2. upon generation, clearly display and label the recipient's address (so that they know what to submit for their airdrop)
3. display the private key as a mnemonic and direct the user to keep it safe (ie. able to recover it; do not compromise it; your keys are your crypto warnings)

these elements assume that we will have an interface for importing mnemonics when Namada mainnet launches (and that the user won't use the CLI)

[gavinly]

i wonder if this should be a standalone binary, instead of being combined with the Namada CLI

[gavinly]

@Fraccaman