Add support for new async api

[rasca]

Django 4.2 added support for async m2m manipulation directly via aadd, aset, aclear, aremove`. Since taggit is based in that API it would be nice to also support it.

For now we can just:

from channels.db import database_sync_to_async

[...]

await database_sync_to_async(mymodel.tags.add)('mytag')

[rtpg]

Not sure what we need to do exactly here, I would hope that models.Manager would get us this "for free", but seems like it might not. We'll likely need to do some work to add some tests on this front as well. Bit of a chill weekend commit, probably!