Add codeowners entries for each part of the provider (#1290)

Author: julienduchesne

.github/CODEOWNERS

@@ -1 +1,8 @@
 * @grafana/terraform-provider
+
+/internal/resources/grafana/*_alerting_* @grafana/terraform-provider @grafana/alerting-squad
+/internal/resources/cloud/* @grafana/terraform-provider @grafana/grafana-com-team
+/internal/resources/machinelearning/* @grafana/terraform-provider @grafana/machine-learning
+/internal/resources/oncall/* @grafana/terraform-provider @grafana/grafana-oncall
+/internal/resources/slo/* @grafana/terraform-provider @grafana/slo-squad
+/internal/resources/syntheticmonitoring/* @grafana/terraform-provider @grafana/synthetic-monitoring