Allow custom headers in validator client HTTP requests (#15884)

* Allow custom headers in validator client HTTP requests

* changelog <3

* improve flag description

* Bastin's review

* James' review

* add godoc for NodeConnectionOption

cmd/validator/flags/flags.go

@@ -45,6 +45,13 @@ var (
 		Usage: "Beacon node REST API provider endpoint.",
 		Value: "http://127.0.0.1:3500",
 	}
+	// BeaconRESTApiHeaders defines a list of headers to send with all HTTP requests to the beacon node.
+	BeaconRESTApiHeaders = &cli.StringFlag{
+		Name: "beacon-rest-api-headers",
+		Usage: `Comma-separated list of key value pairs to pass as headers for all HTTP calls to the beacon node. 
+		To provide multiple values for the same key, specify the same key for each value. 
+		Example: --grpc-headers=key1=value1,key1=value2,key2=value3`,
+	}
 	// CertFlag defines a flag for the node's TLS certificate.
 	CertFlag = &cli.StringFlag{
 		Name:  "tls-cert",

cmd/validator/main.go

@@ -51,6 +51,7 @@ func startNode(ctx *cli.Context) error {
 var appFlags = []cli.Flag{
 	flags.BeaconRPCProviderFlag,
 	flags.BeaconRESTApiProviderFlag,
+	flags.BeaconRESTApiHeaders,
 	flags.CertFlag,
 	flags.GraffitiFlag,
 	flags.DisablePenaltyRewardLogFlag,

cmd/validator/usage.go

@@ -93,6 +93,7 @@ var appHelpFlagGroups = []flagGroup{
 		Flags: []cli.Flag{
 			flags.CertFlag,
 			flags.BeaconRPCProviderFlag,
+			flags.BeaconRESTApiHeaders,
 			flags.EnableRPCFlag,
 			flags.RPCHost,
 			flags.RPCPort,