Plugins: Refactor plugin dashboards (#44315)

Moves/refactor Grafana specific functionality related to plugin dashboards 
out to specific services for importing dashboards and keep app plugin dashboards
up-to-date.

Fixes #44257

pkg/api/dtos/plugins.go

@@ -1,7 +1,6 @@
 package dtos
 
 import (
-	"github.com/grafana/grafana/pkg/components/simplejson"
 	"github.com/grafana/grafana/pkg/plugins"
 )
 
@@ -59,16 +58,6 @@ func (slice PluginList) Swap(i, j int) {
 	slice[i], slice[j] = slice[j], slice[i]
 }
 
-type ImportDashboardCommand struct {
-	PluginId  string                         `json:"pluginId"`
-	Path      string                         `json:"path"`
-	Overwrite bool                           `json:"overwrite"`
-	Dashboard *simplejson.Json               `json:"dashboard"`
-	Inputs    []plugins.ImportDashboardInput `json:"inputs"`
-	FolderId  int64                          `json:"folderId"`
-	FolderUid string                         `json:"folderUid"`
-}
-
 type InstallPluginCommand struct {
 	Version string `json:"version"`
 }