feat(experimental): adds transaction started and transaction closed hooks to the middleware.

[jcchavezs]

This is specially useful when you want to use transaction data to populate things like logs or spans in a trace enriching the experience and connecting the dots with observability.

This is still ongoing. I am not 100% sure the TransactionClose method has any utility besides e.g. adding a span event for start and close or do something with MatchedRules maybe? (request for comments @anuraaga @jptosso @M4tteoP @MrWako)