Add support for using CloudWatch 'Logs Insights' query type when defining a dashboard variable

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: When creating a dashboard based on the CloudWatch Logs data source, we need to use a template variable that gets its values from the results of a CloudWatch Logs Insights query. For example, we could use a query like this to return a list of services, which would be the values of the template variable dropdown. This would allow us to update the dashboards to only display info for the selected service. ``` fields @timestamp, @message | stats count(*) as count by service_name | sort service_name asc ``` **What would you like to be added**: When defining a dashboard variable, and choosing a CloudWatch data source, we should see 'Logs Insights' as an option in the Query Type dropdown. And then be able to enter the desired query which will return the values for that template variable. **Who is this feature for?** Anyone creating monitoring dashboards based on CloudWatch logs.