Restore block_hash to Batch

It's not only helpful (to easily check where Serai's view of the external
network is) but it's necessary in case of a non-trivial chain fork to determine
which blockchain Serai considers canonical.

substrate/in-instructions/primitives/src/lib.rs

@@ -106,6 +106,7 @@ pub struct InInstructionWithBalance {
 pub struct Batch {
   pub network: NetworkId,
   pub id: u32,
+  pub external_network_block_hash: [u8; 32],
   pub instructions: Vec<InInstructionWithBalance>,
 }