Auth: add IsServiceAccount to IsRealUser (#58015)

* add: IsServiceAccount to SignedInUser and IsRealUser

* fix: linting error

* refactor: add function IsServiceAccountUser()

By adding the function IsServiceAccountUser() we use it to identify for
ServiceAccounts in the HasUniqueID() since caching is built up on having
a uniqueID, see comment: https://github.com/grafana/grafana/pull/58015#discussion_r1011361880

pkg/services/querylibrary/tests/common.go

@@ -38,11 +38,12 @@ func createServiceAccountAdminToken(t *testing.T, name string, env *server.TestE
 	})
 
 	return keyGen.ClientSecret, &user.SignedInUser{
-		UserID: account.ID,
-		Email:  account.Email,
-		Name:   account.Name,
-		Login:  account.Login,
-		OrgID:  account.OrgID,
+		UserID:           account.ID,
+		Email:            account.Email,
+		Name:             account.Name,
+		Login:            account.Login,
+		OrgID:            account.OrgID,
+		IsServiceAccount: true,
 	}
 }