Add new MessageOneof rule

proto/protovalidate-testing/buf/validate/conformance/cases/messages.proto

@@ -58,3 +58,34 @@ message MessageRequiredOneof {
 }
 
 message MessageWith3dInside {}
+
+message MessageOneofSingleField {
+  option (buf.validate.message).oneof = {
+    fields: ["str_field"]
+  };
+  string str_field = 1;
+  bool bool_field = 2;
+}
+
+message MessageOneofMultipleFields {
+  option (buf.validate.message).oneof = {
+    fields: [
+      "str_field",
+      "bool_field"
+    ]
+  };
+  string str_field = 1;
+  bool bool_field = 2;
+}
+
+message MessageOneofMultipleFieldsRequired {
+  option (buf.validate.message).oneof = {
+    fields: [
+      "str_field",
+      "bool_field"
+    ]
+    required: true
+  };
+  string str_field = 1;
+  bool bool_field = 2;
+}

proto/protovalidate/buf/validate/validate.proto

@@ -137,6 +137,26 @@ message MessageRules {
   // }
   // ```
   repeated Rule cel = 3;
+
+  // `oneof` is a repeated field of type MessageOneofRule that specifies a list of fields
+  // of which exactly one can be present. If `required` is also specified, then exactly one
+  // of the specified fields _must_ be present.
+  //
+  //
+  // ```proto
+  // message MyMessage {
+  //   // Only one of 'field1' or 'field2' _can_ be present in this message.
+  //   option (buf.validate.message).oneof = { fields: ["field1", "field2"] };
+  //   // Only one of 'field3' or 'field4' _must_ be present in this message.
+  //   option (buf.validate.message).oneof = { fields: ["field3", "field4"], required: true };
+  // }
+  // ```
+  repeated MessageOneofRule oneof = 4;
+}
+
+message MessageOneofRule {
+  repeated string fields = 1;
+  optional bool required = 2;
 }
 
 // The `OneofRules` message type enables you to manage rules for