[DOP-23122] Use async methods of Keycloak client

[dolfinus]

Change Summary

• Do not call blocking methods of Keycloak client inside async def methods of AuthProvider.
• Fix annotations for get_current_user method.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
syncmaster/server/providers/auth
dummy_provider.py	59	22	62%	29–30, 57–58, 60–65, 67–69, 71–73, 80–81, 85, 90, 100–101
keycloak_provider.py	81	15	81%	28–30, 70–71, 102–104, 107, 140–141, 145–148
TOTAL	4772	272	94%