What is this feature?
Add a config option to set data source to imported rules when X-Grafana-Alerting-Datasource-UID is not present.
Why do we need this feature?
Currently mimirtool requires passing --extra-headers 'X-Grafana-Alerting-Datasource-UID: {uid}' when used with Grafana. This config option allows to specify a default, which is used when the header is missing, making it easier to use and more similar to the case when it's used with Mimir.