feat: backend support for creating and storing service accounts (#22698)

Add is_service_account column to users table with CHECK constraints
enforcing login_type='none' and empty email for service accounts.
Update user creation API to validate service account constraints.

Related to:
https://linear.app/codercom/issue/PLAT-27/feat-backend-support-for-creating-and-storing-service-accounts

enterprise/coderd/workspaces_test.go

@@ -4577,6 +4577,7 @@ func TestWorkspacesSharedWith(t *testing.T) {
 		// Update a shared with user to have a name and avatar
 		_, err := db.UpdateUserProfile(dbauthz.AsSystemRestricted(ctx), database.UpdateUserProfileParams{
 			ID:        sharedWithUser.ID,
+			Email:     sharedWithUser.Email,
 			Username:  sharedWithUser.Username,
 			Name:      "Shared User Name",
 			AvatarURL: "/emojis/1fae1.png",
@@ -4664,6 +4665,7 @@ func TestWorkspacesSharedWith(t *testing.T) {
 		// Update a shared with user to have a name and avatar
 		_, err := db.UpdateUserProfile(dbauthz.AsSystemRestricted(ctx), database.UpdateUserProfileParams{
 			ID:        sharedWithUser.ID,
+			Email:     sharedWithUser.Email,
 			Username:  sharedWithUser.Username,
 			Name:      "Shared User Name",
 			AvatarURL: "/emojis/1fae1.png",