Allow exclusion of specific emails from being logged

Problem/Motivation

I am investigating using this module. From what I can see, there is only the option to log all outgoing emails, or none.
It would be valuable if we would be able to add a denylist setting that allows us to exclude certain mails from being logged / traced.

E.g. do not log the notification mail sent by update.module

Proposed resolution

- Add a "denylist" config key that contains a list of mailkeys (with wildcards?) to be excluded from logging
- Update logging logic to take denylist into account

Remaining tasks

- Decide if useful
- Create an MR and review

User interface changes

None

API changes

None

Data model changes

None