vsphere dashboards variable update (DataDog#22164)

* Modernize vSphere dashboards with enhanced filtering and query format

- Update dashboard query format from legacy to modern formula-based queries
- Add comprehensive template variables (vcenter_server, vcenter_datacenter, vsphere_host, VM)
- Enhance filtering capabilities across all widgets with new template variables
- Add vSAN cluster monitoring widgets to overview dashboard
- Standardize dashboard file naming (fix hyphen inconsistency)
- Update manifest.json to reflect new dashboard paths

* Update vsphere/assets/dashboards/vmware_vsphere_-_overview.json

Co-authored-by: Joe Peeples <joe.peeples@datadoghq.com>

* Update vsphere/assets/dashboards/vmware_vsphere_-_property_metrics.json

Co-authored-by: Joe Peeples <joe.peeples@datadoghq.com>

* Update vsphere dashboard filenames and manifest.json

- Rename vmware_vsphere_-_overview.json to vsphere_overview.json
- Rename vmware_vsphere_-_property_metrics.json to vmware_vsphere-_property_metrics.json (fix hyphen)
- Update manifest.json to reflect new dashboard paths

* Add VM template variable filtering to VM widgets and remove layout_type

* Add layout_type to vsphere dashboard

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Co-authored-by: Joe Peeples <joe.peeples@datadoghq.com>

Author: mahipdeora25