Block rewards API endpoint (#12020)

Co-authored-by: terencechain <terence@prysmaticlabs.com>
Co-authored-by: prylabs-bulldozer[bot] <58059840+prylabs-bulldozer[bot]@users.noreply.github.com>

network/BUILD.bazel

@@ -6,6 +6,7 @@ go_library(
         "auth.go",
         "endpoint.go",
         "external_ip.go",
+        "writer.go",
     ],
     importpath = "github.com/prysmaticlabs/prysm/v4/network",
     visibility = ["//visibility:public"],
@@ -13,6 +14,7 @@ go_library(
         "//network/authorization:go_default_library",
         "@com_github_golang_jwt_jwt_v4//:go_default_library",
         "@com_github_pkg_errors//:go_default_library",
+        "@com_github_sirupsen_logrus//:go_default_library",
     ],
 )
 

network/writer.go

@@ -0,0 +1,41 @@
+package network
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"net/http"
+	"strconv"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// DefaultErrorJson is a JSON representation of a simple error value, containing only a message and an error code.
+type DefaultErrorJson struct {
+	Message string `json:"message"`
+	Code    int    `json:"code"`
+}
+
+// WriteJson writes the response message in JSON format.
+func WriteJson(w http.ResponseWriter, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusOK)
+	if err := json.NewEncoder(w).Encode(v); err != nil {
+		log.WithError(err).Error("Could not write response message")
+	}
+}
+
+// WriteError writes the error by manipulating headers and the body of the final response.
+func WriteError(w http.ResponseWriter, errJson *DefaultErrorJson) {
+	j, err := json.Marshal(errJson)
+	if err != nil {
+		log.WithError(err).Error("Could not marshal error message")
+		return
+	}
+	w.Header().Set("Content-Length", strconv.Itoa(len(j)))
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(errJson.Code)
+	if _, err := io.Copy(w, io.NopCloser(bytes.NewReader(j))); err != nil {
+		log.WithError(err).Error("Could not write error message")
+	}
+}