Add option for private notes on `mint` command

Currently the mint command always generates public notes. We should add an optional parameter to generate private notes. This would need to be handled in the auto-consumption by downloading the note from the faucet API and importing it to the client. Or, if the note transport is active, syncing would be enough.

Context: https://github.com/0xMiden/miden-faucet/pull/215#discussion_r2737692319