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.go

@@ -745,26 +745,6 @@ func collectPluginFilesWithin(rootDir string) ([]string, error) {
 	return files, err
 }
 
-// GetDataPlugin gets a DataPlugin with a certain name. If none is found, nil is returned.
-//nolint: staticcheck // plugins.DataPlugin deprecated
-func (pm *PluginManager) GetDataPlugin(id string) plugins.DataPlugin {
-	pm.pluginsMu.RLock()
-	defer pm.pluginsMu.RUnlock()
-
-	if p := pm.GetDataSource(id); p != nil && p.CanHandleDataQueries() {
-		return p
-	}
-
-	// XXX: Might other plugins implement DataPlugin?
-
-	p := pm.BackendPluginManager.GetDataPlugin(id)
-	if p != nil {
-		return p.(plugins.DataPlugin)
-	}
-
-	return nil
-}
-
 func (pm *PluginManager) StaticRoutes() []*plugins.PluginStaticRoute {
 	return pm.staticRoutes
 }

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