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

api/client/BUILD.bazel

@@ -6,6 +6,7 @@ go_library(
         "client.go",
         "errors.go",
         "options.go",
+        "transport.go",
     ],
     importpath = "github.com/OffchainLabs/prysm/v6/api/client",
     visibility = ["//visibility:public"],
@@ -14,7 +15,13 @@ go_library(
 
 go_test(
     name = "go_default_test",
-    srcs = ["client_test.go"],
+    srcs = [
+        "client_test.go",
+        "transport_test.go",
+    ],
     embed = [":go_default_library"],
-    deps = ["//testing/require:go_default_library"],
+    deps = [
+        "//testing/assert:go_default_library",
+        "//testing/require:go_default_library",
+    ],
 )

api/client/transport.go

@@ -0,0 +1,25 @@
+package client
+
+import "net/http"
+
+// CustomHeadersTransport adds custom headers to each request
+type CustomHeadersTransport struct {
+	base    http.RoundTripper
+	headers map[string][]string
+}
+
+func NewCustomHeadersTransport(base http.RoundTripper, headers map[string][]string) *CustomHeadersTransport {
+	return &CustomHeadersTransport{
+		base:    base,
+		headers: headers,
+	}
+}
+
+func (t *CustomHeadersTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	for header, values := range t.headers {
+		for _, value := range values {
+			req.Header.Add(header, value)
+		}
+	}
+	return t.base.RoundTrip(req)
+}

api/client/transport_test.go

@@ -0,0 +1,25 @@
+package client
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/OffchainLabs/prysm/v6/testing/assert"
+	"github.com/OffchainLabs/prysm/v6/testing/require"
+)
+
+type noopTransport struct{}
+
+func (*noopTransport) RoundTrip(*http.Request) (*http.Response, error) {
+	return nil, nil
+}
+
+func TestRoundTrip(t *testing.T) {
+	tr := &CustomHeadersTransport{base: &noopTransport{}, headers: map[string][]string{"key1": []string{"value1", "value2"}, "key2": []string{"value3"}}}
+	req := httptest.NewRequest("GET", "http://foo", nil)
+	_, err := tr.RoundTrip(req)
+	require.NoError(t, err)
+	assert.DeepEqual(t, []string{"value1", "value2"}, req.Header.Values("key1"))
+	assert.DeepEqual(t, []string{"value3"}, req.Header.Values("key2"))
+}