Offline signing: Store public keys in address book

[matevz]

Currently, there is no way to prepare an unsigned transaction using CLI without having the signer's private key. You can only store and use the private key from your wallet and in the address book, only addresses are stored.

I suggest the following: When adding an entry to the address book, one accepted format could also be a public key. In this case the address would be derived from the public key so it is consistent with other address book entries, but the public key would also be stored along. Then, to generate an unsigned transaction with that public key, --account <some_address_book_entry_pubkey> can be passed.

Alternatively, we could support importing accounts with public keys as accounts instead of address book entries. But this would break the current behavior of the wallet (accounts which you can sign tx with) and address book (accounts which you cannot sign txes with).