Add helper functions to sync auth modules located in salt://_auth to master/extmods/auth

[cro]

What does this PR do?

This PR adds the helper functions necessary to sync auth modules from salt://_auth to /var/cache/salt/master/extmods/auth. Auth modules are already loaded by the loader, so manually putting them in the cache directory would cause Salt to "do the right thing" and load them when needed, there was just no way to get them there with Salt utilities.

What issues does this PR fix or reference?

No filed issue that I could see.

Previous Behavior

salt-run saltutil.sync_all would not copy auth modules. There was no salt-run saltutil.sync_auth at all.

New Behavior

salt-run saltutil.sync_all now copies auth modules to /var/cache/salt/master/extmods/auth where the loader will pick them up. There is a corresponding saltutil.sync_auth as well.

Commits signed with GPG?

Yes

[max-arnold]

It makes sense to update the docs (both the release notes and the module list https://docs.saltstack.com/en/develop/topics/development/modules/#module-types), add some tests and also add the corresponding state module (see #51900 which has a test for that) UPD: sorry, my mistake - a state is not required since it is a runner.

[dwoz]

It makes sense to update the docs (both the release notes and the module list https://docs.saltstack.com/en/develop/topics/development/modules/#module-types), add some tests and also add the corresponding state module (see #51900 which has a test for that).

@cro ping.

[cro]

ping @dwoz. This should be ready now.