Events `2.0`: type name agnostic signature topic

[ascjones]

I agree with @ascjones , the best possible solution is trait TopicName { const NAME: &static str; } and const hash calculation. I think we need to create a low-priority issue to not forget about it. Later, maybe someone will make a const version of blake2, and we can easily implement that.

Originally posted by @xgreenx in https://github.com/paritytech/ink/pull/1827#discussion_r1254895010

Related https://github.com/paritytech/ink/issues/1838