Define event types for Miden protocol

## Summary

Define what types of on-chain events we want and need to support on Miden. This is a prerequisite for building a proper event listener in the client.

This should close the discussion at https://github.com/0xMiden/protocol/discussions/2152.

### Outcome

A clear specification of the event types the protocol will support, which will inform the client-side event listener implementation.