[build] create xds_end2end_test_lib visibility tag (#36187)

As title

Closes #36187

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36187 from apolcyn:vis_tag 6ce3930e3f
PiperOrigin-RevId: 619955425

bazel/grpc_build_system.bzl

@@ -120,6 +120,7 @@ def _update_visibility(visibility):
         "tsi": PRIVATE,
         "xds": PRIVATE,
         "xds_client_core": PRIVATE,
+        "xds_end2end_test_utils": PRIVATE,
         "grpc_python_observability": PRIVATE,
         "event_engine_base_hdrs": PRIVATE,
         "useful": PRIVATE,

test/cpp/end2end/xds/BUILD

@@ -25,6 +25,7 @@ grpc_cc_library(
     name = "xds_server",
     srcs = ["xds_server.cc"],
     hdrs = ["xds_server.h"],
+    visibility = ["@grpc:xds_end2end_test_utils"],
     deps = [
         "//:gpr",
         "//:grpc",
@@ -45,6 +46,7 @@ grpc_cc_library(
     name = "xds_utils",
     srcs = ["xds_utils.cc"],
     hdrs = ["xds_utils.h"],
+    visibility = ["@grpc:xds_end2end_test_utils"],
     deps = [
         ":xds_server",
         "//src/proto/grpc/testing/xds/v3:cluster_proto",