Plugins: Renamed parts of the UI extension APIs (#63070)

* Renamed target -> id and href -> path after feedback.

* fixed type issues in test page.

* chore(pluginschemajson): update extensions props target -> id

* this is the final.

* fixed typings...again...

---------

Co-authored-by: Jack Westbrook <jack.westbrook@gmail.com>

pkg/plugins/manager/testdata/test-app-with-link-extensions/plugin.json

@@ -36,14 +36,14 @@
   ],
   "extensions": [
     {
-      "target": "plugins/grafana-slo-app/slo-breach",
+      "placement": "plugins/grafana-slo-app/slo-breach",
       "type": "link",
       "title": "Declare incident",
       "description": "Declares a new incident",
       "path": "/incidents/declare"
     },
     {
-      "target": "plugins/grafana-slo-app/slo-breach",
+      "placement": "plugins/grafana-slo-app/slo-breach",
       "type": "link",
       "title": "Declare incident",
       "description": "Declares a new incident (path without backslash)",