enable commands on locked envelope

[mattrighetti]

Follow up of #39

envelope currently supports encryption with lock and unlock commands, but won't let you run commands if envelope is in a locked state.

This feature aims to enable users to run any available envelope command even when the database is locked.

Notes:

• envelope must be loaded in memory and not persisted in a unencrypted state locally (if for some reason we incur in a failure when the database is unencrypted, we'd inadvertently expose the database locally)
• upon execution, each command would prompt users for a password

[mattrighetti]

Implementation details:

• when envelope is in a locked state we can ask users for a password
• we can use that password to get the unencrypted bytes of the sqlite file
• we can load those bytes in a :memory: sqlite connection and perform the op (command) on that (wrote a little example on how to do that here)
• when the op is completed we can serialize the bytes back
• finally, persist the encrypted serialized bytes on disk