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/client/offline_client.rs

@@ -7,6 +7,7 @@ use crate::{
     constants::ConstantsClient,
     events::EventsClient,
     rpc::types::RuntimeVersion,
+    runtime_api::RuntimeApiClient,
     storage::StorageClient,
     tx::TxClient,
     Config,
@@ -49,6 +50,11 @@ pub trait OfflineClientT<T: Config>: Clone + Send + Sync + 'static {
     fn blocks(&self) -> BlocksClient<T, Self> {
         BlocksClient::new(self.clone())
     }
+
+    /// Work with runtime API.
+    fn runtime_api(&self) -> RuntimeApiClient<T, Self> {
+        RuntimeApiClient::new(self.clone())
+    }
 }
 
 /// A client that is capable of performing offline-only operations.