IAM: Create and delete user from the legacy store (#107694)

* Add Create for User + DualWriter setup

* Add delete User

* Fix delete + access check

* Add tests for delete user

* Add tests for create user

* Fixes

* Use sqlx session to fix database locked issues

* wip authz checks

* legacyAccessClient

* Update legacyAccessClient, add tests for create user

* Close rows before running other queries

* Use ExecWithReturningId

* Verify deletion in the tests

* Add Validate and Mutate

* Other changes

* Address feedback

* Update tests

---------

Co-authored-by: Gabriel Mabille <gabriel.mabille@grafana.com>

pkg/registry/apis/iam/legacy/delete_user.sql

@@ -0,0 +1,4 @@
+-- Delete from user table (org_user will be handled separately to avoid locking)
+DELETE FROM {{ .Ident .UserTable }} 
+WHERE uid = {{ .Arg .Query.UID }} 
+  AND org_id = {{ .Arg .Query.OrgID }}