Chore: Refactor usage of legacy data contracts (#41218)

Refactor usage of legacy data contracts. Moves legacy data contracts 
to pkg/tsdb/legacydata package.
Refactor pkg/expr to be a proper service/dependency that can be provided 
to wire to remove some unneeded dependencies to SSE in ngalert and other places.
Refactor pkg/expr to not use the legacydata,RequestHandler and use 
backend.QueryDataHandler instead.

pkg/plugins/ifaces.go

@@ -10,12 +10,6 @@ import (
 	"github.com/grafana/grafana/pkg/plugins/backendplugin"
 )
 
-// DataRequestHandler is a data request handler interface.
-type DataRequestHandler interface {
-	// HandleRequest handles a data request.
-	HandleRequest(context.Context, *models.DataSource, DataQuery) (DataResponse, error)
-}
-
 // Store is the storage for plugins.
 type Store interface {
 	// Plugin finds a plugin by its ID.