Allow users to create a transaction_t with a signature instead of a private key

[kostaleonard]

As a LeoCoin user, I want miners to be able to create a transaction_t with a signature instead of a private key so that the transaction_t interface doesn't suggest I should be giving them my private key to add the transaction to the next block.

Miners who pick up the transaction would be responsible for verifying the signature before adding it to their next block to mine. Also note that this issue affects blockchain_deserialize, which has to manually alloc a transaction_t instead of using transaction_create.