Introduce the Transform struct transforming CSVs

This allows us to:
  - transform a CSV, a JSON or a JSON lines data type into the same
    Grenad x Obkv streamable data type and creates the new FieldsIdsMap.
  - Extract all the documents user ids in advance to be able to delete
    the existing documents before re-indexing them.
  - Keep the last documents with the same user id avoiding duplicates
    in the same request.

src/indexing/mod.rs

@@ -21,8 +21,9 @@ use self::merge_function::{
     docid_word_positions_merge, documents_merge,
 };
 
-mod store;
 mod merge_function;
+mod store;
+mod transform;
 
 #[derive(Debug, Clone, StructOpt)]
 pub struct IndexerOpt {