REST VC: Subscribe to Beacon API events (#13453)

* Revert "Revert "REST VC: Subscribe to Beacon API events  (#13354)" (#13428)"

This reverts commit 8d092a1113.

* change logic

* review

* test fix

* fix critical error

* merge flag check

* change error msg

* return on errors

---------

Co-authored-by: james-prysm <90280386+james-prysm@users.noreply.github.com>

validator/node/BUILD.bazel

@@ -43,6 +43,7 @@ go_library(
         "//validator:__subpackages__",
     ],
     deps = [
+        "//api:go_default_library",
         "//api/gateway:go_default_library",
         "//api/server:go_default_library",
         "//async/event:go_default_library",

validator/node/node.go

@@ -26,6 +26,7 @@ import (
 	gwruntime "github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
 	"github.com/pkg/errors"
 	fastssz "github.com/prysmaticlabs/fastssz"
+	"github.com/prysmaticlabs/prysm/v4/api"
 	"github.com/prysmaticlabs/prysm/v4/api/gateway"
 	"github.com/prysmaticlabs/prysm/v4/api/server"
 	"github.com/prysmaticlabs/prysm/v4/async/event"
@@ -865,7 +866,7 @@ func (c *ValidatorClient) registerRPCGatewayService(router *mux.Router) error {
 			},
 		}),
 		gwruntime.WithMarshalerOption(
-			"text/event-stream", &gwruntime.EventSourceJSONPb{}, // TODO: remove this
+			api.EventStreamMediaType, &gwruntime.EventSourceJSONPb{}, // TODO: remove this
 		),
 		gwruntime.WithForwardResponseOption(gateway.HttpResponseModifier),
 	)