Extends ChildStateBackend and ChildStateAPI with ReadProofs (#8812)

* Extends ChildStateBackend and ChildStateAPI with ReadProofs

The following changes integrate the existing `read_child_proof`
from the `ProofProvider` into the ChildStateBackend, so that a
read proof can be generated from a full client via an rpc call.

* Cleanup formatting

substrate/client/rpc-api/src/child_state/mod.rs

@@ -23,6 +23,7 @@ use sp_core::storage::{StorageKey, PrefixedStorageKey, StorageData};
 use crate::state::error::FutureResult;
 
 pub use self::gen_client::Client as ChildStateClient;
+use crate::state::ReadProof;
 
 /// Substrate child state API
 ///
@@ -68,4 +69,13 @@ pub trait ChildStateApi<Hash> {
 		key: StorageKey,
 		hash: Option<Hash>
 	) -> FutureResult<Option<u64>>;
+
+	/// Returns proof of storage for child key entries at a specific block's state.
+	#[rpc(name = "state_getChildReadProof")]
+	fn read_child_proof(
+		&self,
+		child_storage_key: PrefixedStorageKey,
+		keys: Vec<StorageKey>,
+		hash: Option<Hash>,
+	) -> FutureResult<ReadProof<Hash>>;
 }