Various simplifications re: Serai transactions

Removes PairSigner for the pair directly.

Resets spec_version to 1.

Defines the extrinsic without its length prefix, only prefixing during publish.

substrate/client/tests/common/tx.rs

@@ -34,7 +34,7 @@ pub async fn publish_tx(serai: &Serai, tx: &[u8]) -> [u8; 32] {
     };
 
     for transaction in block.transactions() {
-      if transaction.0 == tx[2 ..] {
+      if transaction.0 == tx {
         return block.hash();
       }
     }