rpc/client: Propagate rpc_methods method to reported methods (#1713)

The PR exposes the `rpc_methods` name in the `rpc_methods` response.
This feature is useful for servers that only forward requests to methods
that are reported by the `rpc_methods`.

Jsonrpsee exposes the available methods registered so far. Registering
the `rpc_methods` requires a closure that already stores the result. As
such, the `rpc_methods` method name is manually added to the available
methods.

Closes: #1627

### Testing Done

```
$> curl -H "Content-Type: application/json" -d '{"id":1, "jsonrpc":"2.0", "method": "rpc_methods", "params":[]}' http://localhost:9944

{"jsonrpc":"2.0","result":{"methods":["account_nextIndex",... "rpc_methods",..."unsubscribe_newHead"]},"id":1}⏎
```

@paritytech/subxt-team

---------

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

Author: lexnv

substrate/client/rpc-servers/src/lib.rs

@@ -145,6 +145,8 @@ fn hosts_filtering(enabled: bool, addrs: &[SocketAddr]) -> AllowHosts {
 
 fn build_rpc_api<M: Send + Sync + 'static>(mut rpc_api: RpcModule<M>) -> RpcModule<M> {
 	let mut available_methods = rpc_api.method_names().collect::<Vec<_>>();
+	// The "rpc_methods" is defined below and we want it to be part of the reported methods.
+	available_methods.push("rpc_methods");
 	available_methods.sort();
 
 	rpc_api