* Add "synthetics-*-*" templates for synthetics fleet data
For the Elastic Agent we currently have `logs` and `metrics`, however, synthetic data doesn't belong
with those and thus we should have a place for it to live. This would be data reported from
heartbeat and under the 'monitoring' category.
This commit adds a composable index template for `synthetics-*-*` indices similar to the work in
#56709 and #57629.
Resolves#61665
6b2af30a62