[datadog_dashboard] Add description field to widget definitions

[SarthakC]

Summary

Closes #3595

The Datadog dashboard UI supports adding a description to individual widgets (shown as a tooltip on the widget's ? icon). This PR exposes that field in the Terraform provider for all applicable widget definition types.

• The Datadog REST API already supports description inside each widget's definition JSON object — confirmed via GET /api/v1/dashboard/{id}. The Go API client surfaces it through AdditionalProperties since it's not yet in the typed spec.
• Added getWidgetDescriptionSchema() helper returning an optional TypeString field
• Updated 27 widget definition types: schema, buildDatadog*Definition, and buildTerraform*Definition for each
• Skipped container widgets (group, powerpack, split_graph) and text-only widgets (note, free_text)

Example usage:

widget {
  query_value_definition {
    title       = "Active Users"
    description = "Unique users who had at least one session in the selected time window."
    request { ... }
  }
}

Affected widget types

alert_graph, alert_value, change, check_status, distribution, event_stream, event_timeline, geomap, heatmap, hostmap, image, list_stream, log_stream, manage_status, query_table, query_value, run_workflow, scatterplot, service_level_objective, servicemap, slo_list, timeseries, toplist, topology_map, trace_service, treemap, sunburst

Test plan

• Run acceptance test for datadog_dashboard with a query_value_definition that includes a description
• Verify terraform apply sets the description on the widget in the Datadog UI
• Verify terraform plan shows no drift when description is set
• Verify existing dashboards without descriptions continue to work

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