Implement getField CEL function

I'm proposing this as an eventual replacement (before 1.0) of our hack around the fact that the in identifier is reserved in CEL. We need this for protovalidate-cc especially so we can start removing our cel-cpp patches.

Protovalidate PR: bufbuild/protovalidate#352

Author: John Chadwick

buf/validate/internal/BUILD.bazel

@@ -139,6 +139,7 @@ cc_library(
         "@com_google_absl//absl/status",
         "@com_google_cel_cpp//eval/public:cel_function_adapter",
         "@com_google_cel_cpp//eval/public:cel_function_registry",
+        "@com_google_cel_cpp//eval/public/containers:field_access",
         "@com_google_cel_cpp//eval/public/containers:container_backed_map_impl",
         "@com_googlesource_code_re2//:re2",
     ],

buf/validate/internal/extra_func.cc

@@ -26,6 +26,7 @@
 #include "eval/public/cel_function_adapter.h"
 #include "eval/public/cel_value.h"
 #include "eval/public/containers/container_backed_map_impl.h"
+#include "eval/public/containers/field_access.h"
 #include "google/protobuf/arena.h"
 #include "re2/re2.h"
 
@@ -52,6 +53,32 @@ bool isPathValid(const std::string_view path) {
 
 namespace cel = google::api::expr::runtime;
 
+cel::CelValue getField(
+    google::protobuf::Arena* arena, cel::CelValue msgval, cel::CelValue nameval) {
+  if (!msgval.IsMessage()) {
+    auto* error = google::protobuf::Arena::Create<cel::CelError>(
+        arena, absl::StatusCode::kInvalidArgument, "expected a message value for first argument");
+    return cel::CelValue::CreateError(error);
+  }
+  if (!nameval.IsString()) {
+    auto* error = google::protobuf::Arena::Create<cel::CelError>(
+        arena, absl::StatusCode::kInvalidArgument, "expected a string value for second argument");
+    return cel::CelValue::CreateError(error);
+  }
+  const auto* message = msgval.MessageOrDie();
+  auto name = nameval.StringOrDie();
+  const auto* field = message->GetDescriptor()->FindFieldByName(name.value());
+  if (field == nullptr) {
+    auto* error = google::protobuf::Arena::Create<cel::CelError>(
+        arena, absl::StatusCode::kInvalidArgument, "no such field");
+    return cel::CelValue::CreateError(error);
+  }
+  if (cel::CelValue result; cel::CreateValueFromSingleField(message, field, arena, &result).ok()) {
+    return result;
+  }
+  return cel::CelValue::CreateNull();
+}
+
 cel::CelValue isNan(google::protobuf::Arena* arena, cel::CelValue rhs) {
   if (!rhs.IsDouble()) {
     auto* error = google::protobuf::Arena::Create<cel::CelError>(
@@ -351,6 +378,11 @@ absl::Status RegisterExtraFuncs(
           &registry);
   if (!status.ok()) {
     return status;
+  auto getFieldStatus =
+      cel::FunctionAdapter<cel::CelValue, cel::CelValue, cel::CelValue>::CreateAndRegister(
+          "getField", false, &getField, &registry);
+  if (!getFieldStatus.ok()) {
+    return getFieldStatus;
   }
   auto isNanStatus = cel::FunctionAdapter<cel::CelValue, cel::CelValue>::CreateAndRegister(
       "isNan", true, &isNan, &registry);