Plugins: Move backend plugin manager to service (#21474)

Moves backend plugin manager to service instead of
global functions in backendplugin package.

Closes #20053

pkg/plugins/models.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 
 	m "github.com/grafana/grafana/pkg/models"
+	"github.com/grafana/grafana/pkg/plugins/backendplugin"
 	"github.com/grafana/grafana/pkg/setting"
 )
 
@@ -32,7 +33,7 @@ func (e PluginNotFoundError) Error() string {
 }
 
 type PluginLoader interface {
-	Load(decoder *json.Decoder, pluginDir string) error
+	Load(decoder *json.Decoder, pluginDir string, backendPluginManager backendplugin.Manager) error
 }
 
 type PluginBase struct {