[Logs Alerting]Provide alert timestamp as context parameter in alert notification body

[mukeshelastic]

When alert notifications are forwarded to a queue, they might go through a few steps before getting parsed and pushed forward to event management systems. This introduces delays, which today makes it hard to understand when an alert was triggered because we don't provide the original timestamp in the notification body.

Add {{context.timestamp}} parameter to threshold alerts which can be used in notification to link to dashboards with appropriate time range or just as a variable to provide context to on call operator about the time window of the alert.