AuthN: Use sync hook to fetch service account

[kalleep]

What is this feature?
Instead of using user service directly we can use the sync hook to get information about service account.

This is needed for when we move hooks and clients out.

Which issue(s) does this PR fix?:
Fixes #

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.