community[patch]: added content_columns option to CSVLoader (#23809)

**Description:** 
Adding a new option to the CSVLoader that allows us to implicitly
specify the columns that are used for generating the Document content.
Currently these are implicitly set as "all fields not part of the
metadata_columns".

In some cases however it is useful to have a field both as a metadata
and as part of the document content.

libs/community/tests/unit_tests/document_loaders/test_csv_loader.py

@@ -108,6 +108,27 @@ class TestCSVLoader:
         # Assert
         assert result == expected_docs
 
+    def test_csv_loader_content_columns(self) -> None:
+        # Setup
+        file_path = self._get_csv_file_path("test_none_col.csv")
+        expected_docs = [
+            Document(
+                page_content="column1: value1\n" "column3: value3",
+                metadata={"source": file_path, "row": 0},
+            ),
+            Document(
+                page_content="column1: value6\n" "column3: value8",
+                metadata={"source": file_path, "row": 1},
+            ),
+        ]
+
+        # Exercise
+        loader = CSVLoader(file_path=file_path, content_columns=("column1", "column3"))
+        result = loader.load()
+
+        # Assert
+        assert result == expected_docs
+
     # utility functions
     def _get_csv_file_path(self, file_name: str) -> str:
         return str(Path(__file__).resolve().parent / "test_docs" / "csv" / file_name)