Add RPC function state_getProof, resolves #1110 (#5649)

* Add RPC function state_getProof, resolves #1110

* Apply suggestions from code review

* Update client/rpc/src/state/state_full.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Update Cargo.lock

* Make block hash optional

* Wrap StorageProof as Bytes

* Add struct ReadProof

* Fix typo

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

substrate/client/rpc-api/src/state/helpers.rs

@@ -0,0 +1,30 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Substrate state API helpers.
+
+use sp_core::Bytes;
+use serde::{Serialize, Deserialize};
+
+/// ReadProof struct returned by the RPC
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ReadProof<Hash> {
+	/// Block hash used to generate the proof
+	pub at: Hash,
+	/// A proof used to prove that storage entries are included in the storage trie
+	pub proof: Vec<Bytes>,
+}