Add an system_syncState RPC method (#7315)

* Add system_syncState RPC method

* Update client/rpc-api/src/system/helpers.rs

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

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

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

@@ -86,6 +86,18 @@ pub enum NodeRole {
 	Sentry,
 }
 
+/// The state of the syncing of the node.
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SyncState<Number> {
+	/// Height of the block at which syncing started.
+	pub starting_block: Number,
+	/// Height of the current best block of the node.
+	pub current_block: Number,
+	/// Height of the highest block learned from the network. Missing if no block is known yet.
+	#[serde(default = "Default::default", skip_serializing_if = "Option::is_none")]
+	pub highest_block: Option<Number>,
+}
 #[cfg(test)]
 mod tests {
 	use super::*;
@@ -114,4 +126,25 @@ mod tests {
 			r#"{"peerId":"2","roles":"a","bestHash":5,"bestNumber":6}"#,
 		);
 	}
+
+	#[test]
+	fn should_serialize_sync_state() {
+		assert_eq!(
+			::serde_json::to_string(&SyncState {
+				starting_block: 12u32,
+				current_block: 50u32,
+				highest_block: Some(128u32),
+			}).unwrap(),
+			r#"{"startingBlock":12,"currentBlock":50,"highestBlock":128}"#,
+		);
+
+		assert_eq!(
+			::serde_json::to_string(&SyncState {
+				starting_block: 12u32,
+				current_block: 50u32,
+				highest_block: None,
+			}).unwrap(),
+			r#"{"startingBlock":12,"currentBlock":50}"#,
+		);
+	}
 }