archive: Implement height, hashByHeight and call (#1582)

This PR implements:
- `archive_unstable_finalized_height`: Get the height of the most recent
finalized block
- `archive_unstable_hash_by_height`: Get the hashes (possible empty) of
blocks from the given height
- `archive_unstable_call`: Call into the runtime of a block

Builds on top of: https://github.com/paritytech/polkadot-sdk/pull/1560

### Testing Done
- unit tests for the methods with custom block tree for different
heights / forks

Closes: https://github.com/paritytech/polkadot-sdk/issues/1510
Closes: https://github.com/paritytech/polkadot-sdk/issues/1513
Closes: https://github.com/paritytech/polkadot-sdk/issues/1511

@paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>

substrate/client/rpc-spec-v2/src/lib.rs

@@ -23,6 +23,8 @@
 #![warn(missing_docs)]
 #![deny(unused_crate_dependencies)]
 
+use serde::{Deserialize, Serialize};
+
 pub mod archive;
 pub mod chain_head;
 pub mod chain_spec;
@@ -30,3 +32,74 @@ pub mod transaction;
 
 /// Task executor that is being used by RPC subscriptions.
 pub type SubscriptionTaskExecutor = std::sync::Arc<dyn sp_core::traits::SpawnNamed>;
+
+/// The result of an RPC method.
+#[derive(Debug, Deserialize, Serialize, PartialEq)]
+#[serde(untagged)]
+pub enum MethodResult {
+	/// Method generated a result.
+	Ok(MethodResultOk),
+	/// Method ecountered an error.
+	Err(MethodResultErr),
+}
+
+impl MethodResult {
+	/// Constructs a successful result.
+	pub fn ok(result: impl Into<String>) -> MethodResult {
+		MethodResult::Ok(MethodResultOk { success: true, result: result.into() })
+	}
+
+	/// Constructs an error result.
+	pub fn err(error: impl Into<String>) -> MethodResult {
+		MethodResult::Err(MethodResultErr { success: false, error: error.into() })
+	}
+}
+
+/// The successful result of an RPC method.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct MethodResultOk {
+	/// Method was successful.
+	success: bool,
+	/// The result of the method.
+	pub result: String,
+}
+
+/// The error result of an RPC method.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct MethodResultErr {
+	/// Method encountered an error.
+	success: bool,
+	/// The error of the method.
+	pub error: String,
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn method_result_ok() {
+		let ok = MethodResult::ok("hello");
+
+		let ser = serde_json::to_string(&ok).unwrap();
+		let exp = r#"{"success":true,"result":"hello"}"#;
+		assert_eq!(ser, exp);
+
+		let ok_dec: MethodResult = serde_json::from_str(exp).unwrap();
+		assert_eq!(ok_dec, ok);
+	}
+
+	#[test]
+	fn method_result_error() {
+		let ok = MethodResult::err("hello");
+
+		let ser = serde_json::to_string(&ok).unwrap();
+		let exp = r#"{"success":false,"error":"hello"}"#;
+		assert_eq!(ser, exp);
+
+		let ok_dec: MethodResult = serde_json::from_str(exp).unwrap();
+		assert_eq!(ok_dec, ok);
+	}
+}