PluginExtensions: Add extension point for overriding Observability home page (#110500)

* feat/add_observability_landing

* Add check for observability path

* Fix existing tests

* Test that we're rendering the component when in the correct path

* Reset all mocks after testing

* Check for extension only on observability route

* Undo changes to tests

* Extract strings to constants

* Remove unused validator

* Remove unnecesary ObservabilityLanding component

* Update subtitle for Observability section

* Use proper '

* Expose extension point, allow plugins to hook into it, and render received components

* Fix and test

* Remove no longer needed unit tests

* Readd validation checks, allow for regex like paths

* refactor(extensions): extract dynamic extension point ids to a separate enum

* Undo unwanted const to let change

* Update extension point id to better transmit intent and use

---------

Co-authored-by: Levente Balogh <balogh.levente.hu@gmail.com>

pkg/services/navtree/navtreeimpl/applinks.go

@@ -236,7 +236,7 @@ func (s *ServiceImpl) addPluginToSection(c *contextmodel.ReqContext, treeRoot *n
 			treeRoot.AddSection(&navtree.NavLink{
 				Text:       "Observability",
 				Id:         navtree.NavIDObservability,
-				SubTitle:   "Opinionated observability across applications, services, and infrastructure",
+				SubTitle:   "Monitor infrastructure and applications in real time with Grafana Cloud's fully managed observability suite",
 				Icon:       "heart-rate",
 				SortWeight: navtree.WeightObservability,
 				Children:   []*navtree.NavLink{appLink},