Subject Aliases

**Describe the solution you'd like**
A way to tag subjects with multiple names that could either live under the original parent subject name or just be like symlinks to the same backend subject id.

In responses that contain the subject there could be an `aka`/`aliases` array that contain a list of the other names that are set to it

**Describe alternatives you've considered**
I have not tried any alternative ways. Unless I've glossed over some docs there doesn't seem to be a way to tag a subject with multiple 'names' unless multiple subjects are created.

**Additional context**
My use case is storing based off usernames, when a user changes their username (or has alternate accounts) it would be nicer to just tag the new name under original subject name instead of renaming the subject and organizing the aliases in a separate database to check for the current subject.