Expose the rpc client in Client (#267)

When we build a `Client` for a substrate chain, we can only access the
methods exposed in `Client` which does not cover all the RPC interfaces
in the chain, it's not feasible to request the methods like `chain_getKeys`
with `Client` only. The downstream user of this library has to build another
`RpcClient` for the uncovered RPC interfaces, which can be unneccessary.

src/rpc.rs

@@ -258,7 +258,8 @@ pub struct ReadProof<Hash> {
 
 /// Client for substrate rpc interfaces
 pub struct Rpc<T: Runtime> {
-    client: RpcClient,
+    /// Rpc client for sending requests.
+    pub client: RpcClient,
     marker: PhantomData<T>,
     accept_weak_inclusion: bool,
 }