Implement QueryablePayload::transaction for Payload (#1761)

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Implements `QueryablePayload::transaction` for `Payload` using
`Payload::transaction`.
Until now `Payload` had only the generic `QueryablePayload::transaction`
which internally uses `transaction_with_proof`. This is not optimal as
proof calculation is cpu-intensive and it's a waste of cpu cycles in
case we only care about the transaction and not the proof.
This change greatly improves the builder's performance.

Tested with `demo-native` it looks like this solves empty blocks issue.

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types/src/v0/impls/block/full_payload/payload.rs

@@ -219,6 +219,14 @@ impl QueryablePayload<SeqTypes> for Payload {
 
         TxProof::new(index, self, &common)
     }
+
+    fn transaction(
+        &self,
+        _meta: &Self::Metadata,
+        index: &Self::TransactionIndex,
+    ) -> Option<Self::Transaction> {
+        self.transaction(index)
+    }
 }
 
 impl std::fmt::Display for Payload {