[ENHANCEMENT] Add grouped variable helper in Go SDK

[Gladorme]

Description

Align Go SDK with Cue feature, you can create grouped variables. Grouped variables will be filtered by their parent variables. For now, it only applied automatically to label values and label names variables matchers.

Prev:

	dashboard.AddVariable("stack",
		listVar.List(
			labelValuesVar.PrometheusLabelValues("stack",
				labelValuesVar.Matchers("thanos_build_info{}"),
				labelValuesVar.Datasource("promDemo"),
			),
			listVar.DisplayName("PaaS"),
		),
	),
	dashboard.AddVariable("namespace", listVar.List(
		labelValuesVar.PrometheusLabelValues("namespace",
			labelValuesVar.Matchers("kube_namespace_labels{\"stack\"=\"$stack\"}"),
			labelValuesVar.Datasource("promDemo"),
		),
	)),
	dashboard.AddVariable("namespaceLabels", listVar.List(
		labelNamesVar.PrometheusLabelNames(
			labelNamesVar.Matchers("kube_namespace_labels{\"stack\"=\"$stack\", \"namespace\"=\"$namespace\"}"),
			labelNamesVar.Datasource("promDemo"),
		),
	)),
	dashboard.AddVariable("pod", listVar.List(
		labelValuesVar.PrometheusLabelValues("pod",
			labelValuesVar.Matchers("kube_namespace_labels{\"stack\"=\"$stack\", \"namespace\"=\"$namespace\"}"),
			labelValuesVar.Datasource("promDemo"),
		),
	)),

New:

dashboard.AddVariableGroup(
	group.AddVariable("stack",
		listVar.List(
			labelValuesVar.PrometheusLabelValues("stack",
				labelValuesVar.Matchers("thanos_build_info"),
				labelValuesVar.Datasource("promDemo"),
			),
			listVar.DisplayName("PaaS"),
		),
	),
	group.AddVariable("namespace", listVar.List(
		labelValuesVar.PrometheusLabelValues("namespace",
			labelValuesVar.Matchers("kube_namespace_labels"),
			labelValuesVar.Datasource("promDemo"),
		),
	)),
	group.AddIgnoredVariable("namespaceLabels", listVar.List(
		labelNamesVar.PrometheusLabelNames(
			labelNamesVar.Matchers("kube_namespace_labels"),
			labelNamesVar.Datasource("promDemo"),
		),
	)),
	group.AddVariable("pod", listVar.List(
		labelValuesVar.PrometheusLabelValues("pod",
			labelValuesVar.Matchers("kube_namespace_labels"),
			labelValuesVar.Datasource("promDemo"),
		),
	)),
),

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.