Namada Keychain: Discussion to migrate keychain (introducing Proof-Gen keys)

[jurevans]

Currently, accounts in Namada Keychain do not have Proof Generation keys. For existing users to access proof-generation keys, they could:

• Re-import their accounts (not ideal)
• Namada Keychain provides a one-time update process when the new keychain is released, where the user authenticates, and we append proof-gen keys for each shielded account (probably the easiest solution for users)
• Generate proof-gen keys on a per-accounts basis, perhaps when they're needed (potentially the most annoying solution)

Also, we should think about how this could work for Ledger as the process is a bit different (since it requires outside approval) - This may be the exception where we ask that they re-import the account, as it's easier than importing a mnemonic.

The migration process (in this case) should only touch type: "shielded-keys", appending the proof-gen keys

• Authenticate to unlock spending key
• From this, generate proof-gen keys
• Append to record

The type: "ledger" accounts will need a corresponding type: "shielded-keys" account entry, where address equals the payment address, and we add viewing key & proof-gen keys, and parentAccount equals the id of the original Ledger account. There could even be a separate import where user selects a Ledger account (to get the path) and they import shielded keys from Ledger.