Add a trie_root_hash variant for chain specs genesis (#10140)

* Add a `hash` variant for chain specs

* Add doc

* Rename to TrieRootHash

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Rustfmt

* More cargo fmt I guess

* Ok I have to use nightly cargo fmt

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/client/chain-spec/src/chain_spec.rs

@@ -119,6 +119,10 @@ impl<G: RuntimeGenesis, E> BuildStorage for ChainSpec<G, E> {
 					})
 					.collect(),
 			}),
+			// The `StateRootHash` variant exists as a way to keep note that other clients support
+			// it, but Substrate itself isn't capable of loading chain specs with just a hash at the
+			// moment.
+			Genesis::StateRootHash(_) => Err("Genesis storage in hash format not supported".into()),
 		}
 	}
 
@@ -144,6 +148,8 @@ pub struct RawGenesis {
 enum Genesis<G> {
 	Runtime(G),
 	Raw(RawGenesis),
+	/// State root hash of the genesis storage.
+	StateRootHash(StorageData),
 }
 
 /// A configuration of a client. Does not include runtime storage initialization.