Chore: Refactor backend plugin manager/tsdb query data (#34944)

Move QueryData method into backend plugin manager which HandleRequest uses to 
query data from plugin SDK supported data sources. This allowed us to remove a lot 
of code no longer needed.

Ref #21510

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

pkg/plugins/manager/manager_test.go

@@ -551,8 +551,6 @@ func verifyBundledPluginCatalogue(t *testing.T, pm *PluginManager) {
 }
 
 type fakeBackendPluginManager struct {
-	backendplugin.Manager
-
 	registeredPlugins []string
 }
 
@@ -566,6 +564,10 @@ func (f *fakeBackendPluginManager) RegisterAndStart(ctx context.Context, pluginI
 	return nil
 }
 
+func (f *fakeBackendPluginManager) Get(pluginID string) (backendplugin.Plugin, bool) {
+	return nil, false
+}
+
 func (f *fakeBackendPluginManager) UnregisterAndStop(ctx context.Context, pluginID string) error {
 	var result []string
 
@@ -600,9 +602,15 @@ func (f *fakeBackendPluginManager) CheckHealth(ctx context.Context, pCtx backend
 	return nil, nil
 }
 
+func (f *fakeBackendPluginManager) QueryData(ctx context.Context, req *backend.QueryDataRequest) (*backend.QueryDataResponse, error) {
+	return nil, nil
+}
+
 func (f *fakeBackendPluginManager) CallResource(pluginConfig backend.PluginContext, ctx *models.ReqContext, path string) {
 }
 
+var _ backendplugin.Manager = &fakeBackendPluginManager{}
+
 type fakePluginInstaller struct {
 	installCount   int
 	uninstallCount int