Refactor contract events

After #1 

Into<Event> and TryFrom<Event> with public consts so external code can work with it programmatically

See [PushMessageEvent](https://github.com/Lay3rLabs/cw-middleware/blob/ec011841419e504e82b5b313aad27f311c740b44/packages/contracts/trigger/api/src/simple.rs#L24) for example