[Proof of Concept] Add support for Kibana Dashboards API **Search** endpoint to enable terraform query / bulk import for dashboards

[nickpeihl]

Add elasticstack_kibana_dashboard list resource to support terraform query bulk import

• Introduces a dashboardsListResource in list.go that calls the Kibana Dashboards Search API via a new SearchDashboards function in dashboards.go, supporting paging, search string, and tag filters.
• The list resource streams results with composite identities (<space_id>/<dashboard_id>) and optionally fetches full resource state per dashboard when include_resource is set.
• Registers the list resource as experimental in plugin_framework.go, enabled only when IncludeExperimentalEnvVar is true or in acceptance tests.
• Adds resource.ResourceWithIdentity support to the dashboard resource so that create, read, and import operations populate Terraform identity alongside state.
• Risk: The list resource and identity support are experimental and gated behind an env var flag; the Kibana Search endpoint is hit once per page, so large dashboards collections will generate many API calls when include_resource is enabled.

^{Macroscope summarized 867674d.}

cc @TinaHeiligers