feat(proto): add write relation to support multiple outputs

proto/substrait/algebra.proto

@@ -142,6 +142,49 @@ message ReadRel {
   }
 }
 
+// This operator passes-through while writing to an external sink
+message WriteRel {
+  RelCommon common = 1;
+  Rel input = 2;
+  substrait.extensions.AdvancedExtension advanced_extension = 10;
+
+  oneof write_type {
+    LocalFiles local_files = 3;
+  }
+
+  message LocalFiles {
+
+    // The consumer should avoid data duplication across items
+    repeated FileOrFiles items = 1;
+    substrait.extensions.AdvancedExtension advanced_extension = 10;
+
+    message FileOrFiles {
+      oneof path_type {
+        string uri_path = 1;
+        string uri_path_glob = 2;
+        string uri_file = 3;
+        string uri_folder = 4;
+      }
+
+      FileFormat format = 5;
+
+      // the index of the partition this item belongs to
+      uint64 partition_index = 6;
+
+      // the project-expressions to apply when writing this item
+      repeated Expression project_expressions;
+
+      // the filter-condition to apply when writing this item
+      Expression filter_condition;
+
+      enum FileFormat {
+        FILE_FORMAT_UNSPECIFIED = 0;
+        FILE_FORMAT_PARQUET = 1;
+      }
+    }
+  }
+}
+
 // This operator allows to represent calculated expressions of fields (e.g., a+b). Direct/Emit are used to represent classical relational projections
 message ProjectRel {
   RelCommon common = 1;
@@ -361,6 +404,7 @@ message Rel {
     ExtensionMultiRel extension_multi = 10;
     ExtensionLeafRel extension_leaf = 11;
     CrossRel cross = 12;
+    WriteRel write = 13;
   }
 }