add ability to pull UID directly

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: Our goal is to avoid manually maintaining the UID in labels, since the rule UID already exists internally in Grafana. We were hoping there might be a way to automatically reference the rule UID in notification templates, for example through a built-in template variable. **What would you like to be added**: Basically, we just don't want to use a label. **Who is this feature for?** The reason we prefer the UID instead of the alert title is related to our AI agent integration. The AI agent processes alert emails and requires a stable and unique identifier to map alerts internally. Using the UID allows the agent to reliably correlate events, such as matching firing and resolved notifications for the same alert rule. For now, we are using a workaround by adding the UID as a label (rule_uid) in the alert rule so it can be included in the email notification.