Basic streaming plugin support (#31940)

This pull request migrates testdata to coreplugin streaming capabilities,
this is mostly a working concept of streaming plugins at the moment, 
the work will continue in the following pull requests.

pkg/tsdb/testdatasource/testdata.go

@@ -33,6 +33,7 @@ func (p *testDataPlugin) Init() error {
 	factory := coreplugin.New(backend.ServeOpts{
 		QueryDataHandler:    p.queryMux,
 		CallResourceHandler: httpadapter.New(resourceMux),
+		StreamHandler:       newTestStreamHandler(p.logger),
 	})
 	err := p.BackendPluginManager.Register("testdata", factory)
 	if err != nil {