Enable Backfill in E2E (#13524)

* enable backfill for devmode

* enable backfill

* gaz

* move to its own package

* fix panic

* fix bug

* gaz

* kasey's review

config/features/BUILD.bazel

@@ -12,6 +12,7 @@ go_library(
     visibility = ["//visibility:public"],
     deps = [
         "//cmd:go_default_library",
+        "//cmd/beacon-chain/sync/backfill/flags:go_default_library",
         "//config/params:go_default_library",
         "@com_github_sirupsen_logrus//:go_default_library",
         "@com_github_urfave_cli_v2//:go_default_library",

config/features/flags.go

@@ -3,6 +3,7 @@ package features
 import (
 	"time"
 
+	backfill "github.com/prysmaticlabs/prysm/v4/cmd/beacon-chain/sync/backfill/flags"
 	"github.com/urfave/cli/v2"
 )
 
@@ -161,6 +162,7 @@ var devModeFlags = []cli.Flag{
 	enableVerboseSigVerification,
 	EnableEIP4881,
 	enableExperimentalState,
+	backfill.EnableExperimentalBackfill,
 }
 
 // ValidatorFlags contains a list of all the feature flags that apply to the validator client.