tracks erasure coding shreds' indices explicitly (#21822)

The indices for erasure coding shreds are tied to data shreds:
https://github.com/solana-labs/solana/blob/90f41fd9b/ledger/src/shred.rs#L921

However with the upcoming changes to erasure schema, there will be more
erasure coding shreds than data shreds and we can no longer infer coding
shreds indices from data shreds.

The commit adds constructs to track coding shreds indices explicitly.

gossip/src/duplicate_shred.rs

@@ -341,6 +341,7 @@ pub(crate) mod tests {
             &entries,
             true, // is_last_in_slot
             next_shred_index,
+            next_shred_index, // next_code_index
         );
         data_shreds.swap_remove(0)
     }