Ledger/transaction_logic: extract zkapp_command module to separate file

Extract the zkapp_command module from transaction_logic/mod.rs into its
own file. This is a large module (~3300 lines) containing zkApp command
types, account updates, events, actions, and verification logic.

Changes:
- Extract zkapp_command module to transaction_logic/zkapp_command.rs
- Use explicit imports instead of 'use super::*'
- Add required trait imports (Zero, Magnitude, Itertools, AppendToInputs)
- Update mod.rs to reference the new module file

Author: dannywillems