feat: add CSV (text) file support (#646)

First version of CSV options (inspired by various engines/libraries)

proto/substrait/algebra.proto

@@ -136,14 +136,41 @@ message ReadRel {
       message ArrowReadOptions {}
       message OrcReadOptions {}
       message DwrfReadOptions {}
+      message DelimiterSeparatedTextReadOptions {
+        // Delimiter separated files may be compressed.  The reader should
+        // autodetect this and decompress as needed.
+
+        // The character(s) used to separate fields.  Common values are comma,
+        // tab, and pipe.  Multiple characters are allowed.
+        string field_delimiter = 1;
+        // The maximum number of bytes to read from a single line.  If a line
+        // exceeds this limit the resulting behavior is undefined.
+        uint64 max_line_size = 2;
+        // The character(s) used to quote strings.  Common values are single
+        // and double quotation marks.
+        string quote = 3;
+        // The number of lines to skip at the beginning of the file.
+        uint64 header_lines_to_skip = 4;
+        // The character used to escape characters in strings.  Backslash is
+        // a common value.  Note that a double quote mark can also be used as an
+        // escape character but the external quotes should be removed first.
+        string escape = 5;
+        // If this value is encountered (including empty string), the resulting
+        // value is null instead.  Leave unset to disable.  If this value is
+        // provided, the effective schema of this file is comprised entirely of
+        // nullable strings.  If not provided, the effective schema is instead
+        // made up of non-nullable strings.
+        optional string value_treated_as_null = 6;
+      }
 
-      // The format of the files.
+      // The format of the files along with options for reading those files.
       oneof file_format {
         ParquetReadOptions parquet = 9;
         ArrowReadOptions arrow = 10;
         OrcReadOptions orc = 11;
         google.protobuf.Any extension = 12;
         DwrfReadOptions dwrf = 13;
+        DelimiterSeparatedTextReadOptions text = 14;
       }
     }
   }