Add Runtime API to execute runtime calls (#777)

* rpc: Add RuntimeAPI call via `state_call` method

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* subxt: Add runtime API

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Expose the RuntimeAPI client

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* tests: Test the runtime API call against the metadata

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* client: Fetch the metadata from runtime API

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* blocks: Fix doc typo

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* blocks: Use &str instead of String to identify fn names

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Update subxt/src/runtime_api/runtime_client.rs

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

subxt/src/rpc/rpc.rs

@@ -371,6 +371,25 @@ impl<T: Config> Rpc<T> {
         Ok(xt_hash)
     }
 
+    /// Execute a runtime API call.
+    pub async fn state_call(
+        &self,
+        function: &str,
+        call_parameters: Option<&[u8]>,
+        at: Option<T::Hash>,
+    ) -> Result<types::Bytes, Error> {
+        let call_parameters = call_parameters.unwrap_or_default();
+
+        let bytes: types::Bytes = self
+            .client
+            .request(
+                "state_call",
+                rpc_params![function, to_hex(call_parameters), at],
+            )
+            .await?;
+        Ok(bytes)
+    }
+
     /// Create and submit an extrinsic and return a subscription to the events triggered.
     pub async fn watch_extrinsic<X: Encode>(
         &self,