Implement tendermint_machine::Block for Substrate Blocks

Unfortunately, this immediately makes Tendermint machine capable of 
deployment as  crate since it uses a git reference. In the future, a 
Cargo.toml patch section for serai/substrate should be investigated. 
This is being done regardless as it's the quickest way forward and this 
is for Serai.

Cargo.lock

@@ -8774,6 +8774,7 @@ version = "0.1.0"
 dependencies = [
  "async-trait",
  "parity-scale-codec",
+ "sp-runtime",
  "tokio",
 ]