Support multi-account functionality.

[nuttycom]

Is your feature request related to a problem? Please describe.

It would be very useful to be able to have the ability to have multiple accounts in addition to the main spending account of the wallet. This includes both "view-only", FVK-only accounts, as well as accounts for which the seed phrase is unknown but for which Zashi provides spending functionality, such as accounts where the UFVK corresponds to a spending key controlled by a hardware wallet.

Describe the solution you'd like

Make it possible to import full and incoming viewing keys to an existing wallet with spending capability. When I import a viewing key, I should be prompted to enter a name for the account. The main screen should include an account switcher that shows these names.

An imported key should be marked as either intended for spending (in which case note commitment tree maintenance is performed) or viewing. Long-term, it would be useful to make it possible to upgrade a viewing account to a spending account.

It should be possible mark a specific account as "sensitive" such that it requires both a PIN and biometric auth to display the balance and history for that account. Cold storage wallets will usually store more value than hot wallets, and so the balance information should be treated as more sensitive.